Holding Fiat money is a total waste of resources and strength in the long-term. The level of inflation happening globally is on the rise and no matter the currency, this inflation is everywhere and it's even worse for some currencies. Choosing bitcoin will be a better option here since you seem to be interested in keeping the amount for a long time.

Saving your money in bitcoin will help you have an edge over inflation and also grants to authority over your money. Over the years we have seen that no fiat has performed better than bitcoin and accepting an 11% interest when you can have over 100% interest or more or less in the long term will not be a good option. It is true that nothing is certain and there's no promises of how much gain your bitcoin would generate, but self custody is gold and you will not face a level of government influence and charges on your money.

Using the rule of 72, which provides a quick method to estimate the time needed for an investment to double, we can determine that 72 ÷ 11.5 indicates it will take approximately 6.3 years for your 11.5% investment in FDR to double, while the 10.5% DPS will require around 6.9 years. However, Bitcoin has showcase outperformed in every cycle, achieving at least 6x returns.
Your concept of using free money to generate more free money is quite intriguing, as some people are blind to it, calling BTC a fraud.

Interest rate can change every year, 6-7 years long to double your investment might probably not enough and it will take you longer or maybe double than that.

Here is the reason why...

Inflation will highly affect this calculation.
Let's say that the inflation rate will be like this:
2026 - 8%
2027 - 9%
2028 - 7.5%
2029 - 8.5%
2030 - 9.5%
2031 - 6.5%
2032 - 5.5%

If your annual interest rate is 11.5%, your real return will be calculated if you less the inflation rate each year to the annual interest rate.
So the earned 11.5% annually doesn't mean you are becoming richer and found a way to beat the system. You definitely increase in numbers, but you are losing in terms of purchasing power.

Let's assume that the average inflation rate for 7years is 8%,
And if you happen to invest 100k in your currency with 11.5% return annually,
That would more than 200k in 6-7years, but it's purchasing power would be around 130k in todays value.

You indeed double your money (in numbers), but the issue is its purchasing power after 7 years.

With the rate of inflation in my country, I doubt if I'll hold a huge amount of money in my bank cause that's a waste of time, it's a better I put my money into an asset that would give it more value with time. Keeping money in the bank means that I'll have to pay them charges for holding my money.

However if my money is stored in Bitcoin, there would be no charges fee deducted from it, I'll gain more as time goes and I'll also have self custody of my own money and use it as it suits me unlike bank that do limit customers daily usuage cause of policies.

You may feel very fortunate to be earning interest on your money through a savings account. But perhaps you haven’t realized that there is a hidden tax on the fiat currency you save in the bank. And you may not be aware that the real value of the fiat currency you hold will erode due to inflation, where you can check the annual inflation rate analysis to see this for yourself (Source). On the other hand the ongoing printing of paper money can also reduce the value of fiat currency. And I think there are still many more factors you should consider to understand the shortcomings of fiat currency. As for Bitcoin we don’t know for sure how much profit will be made in the long term. And that depends on the personal experience of each Bitcoin holder starting from the entry price at which or they began buying it. But the potential for a significant price increase in Bitcoin which is highly likely to occur around the halving cycle could be very profitable roughly every 4 years. There are no hidden taxes associated with holding Bitcoin. And you don’t have to take on double the risk with your Bitcoin because you’re the only one who actually holds it, where there’s no third party involved. Bitcoin’s supply also cannot be printed like fiat currency, where its supply is strictly limited with no strings attached. So I personally believe that Bitcoin is the best choice from every angle. And you also don’t have to worry about a bank potentially going bankrupt.

I know Opay as a good fintech bank but its status is always controversial. I don't know why it's always on the news. Today, it's winding up, tomorrow it's still there. That has made so many Nigerians use them for day to day transactions and not for long term services.

If they give up 15% interest, it's a good one and worthy to be tried.

I can't suggest you to stick to bank or go for bitcoin but I would like to clarify something. There's nothing like free money anywhere. The interest is not actually free as you assumed because you rent out your capital. If its free money, you won't need to lock your capital before earning it.

Moreover, have you considered the inflation rate in those periods before calculating your profits? Because inflation rate can affect your real return at the end of the year. If you're after profit, that's not the right choice but if you're after capital preservation, then you have made the right choice. BTC can give much bigger profit but it is more risky and volatile.

In the bank, you may get double the amount from FDR after 6-7 years, but how much will your fiat currency suffer due to inflation? Will it be in line with inflation? Or will you benefit if you calculate inflation? Inflation is increasing very fast these days, if inflation doubles in 6-7 years, you basically do not get any profit.

In the long term, Bitcoin will protect you from inflation. Many people think that keeping money in the bank is relatively safe, but banks are still far behind in terms of privacy and freedom. Even I am familiar with incidents like bank harassment. Keeping money in the bank is also risky like Bitcoin due to incidents like being cheated by the bank or declaring bankruptcy or getting hacked by the bank. But since banks are a traditional financial system, people have strong faith in banks, which is not the case with Bitcoin.

Excluding inflation, both investment markets seem profitable, you can enter both markets. Since you are already reporting the money received from DPS as free money, I can call that money discretionary money. And Bitcoin investment requires discretionary money. So I don't see anything wrong with investing with the interest received from DPS.

He posted 2 to 3x and not 3x.

Bitcoin is still at $76000 right now, there is a very high probability that bitcoin will get to $152000 and also increase more than that in the next bull run.

Although, I like the other part of your post which let people know the disadvantage of fiat.

Since, I found out about bitcoin and started my bitcoin investment, I don't keep or save money in the bank anymore. All my savings are put into bitcoin using DCA weekly. I love it that way because bitcoin is an asset that increase in value overtime and a long term store of value. While, fiat losses its purchasing power overtime. This makes even your profit from you savings in the bank useless because your profit will diminish overtime even when your funds are still locked. The decentralized nature of bitcoin makes it superb because you have full control over your funds any time and any day.