If you absolutely don't want to use 3rd party software(including open-source), then just simply paste in your wallet address or qr code. Or if you want to be slightly more fancy, add a donate button(on your menu bar or at the bottom of the page) and use a modal pop-up with your wallets address/qrcode when clicked.

Just showing a bitcoin address will discourage someone from tipping you. Those days when you could dip on-chain have gone unless the donator is especially generous.

What you want can be implemented without a third party using lightning payments. Check BTCPay Server.

Nah... I think it's as good as an invoice. Write “Tip me!” instead to avoid seeming like a purchase.

as some friends have said here, using or embedding a Donate Link and a Qr Code that goes directly to your own BTC address will be better than using a third party address.
But the use of addresses should not only include BTC addresses but several other altcoin addresses such as ETH, BNB, SOL, Polygon etc. which have a low fee to send tips that will be given to you.

This is good suggestion and he can simply make changes in CSS to give it a more handy look that doesn't display the address directly and it's hidden and on the click of button it appears on the screen of user in full clarity to avoid any confusions.Even if you don't know about it there are tons of tutorial available from where you can learn how to do it.

I can see there's some dispute over the solution mk4 offered, but I think it's the best way to go. Tips aren't something that should require a payment processor, IMO. A Bitcoin address is the most direct and straightforward way to accept tips, and those who want to give them will be able to do it relatively easily. My concern would perhaps be related to high transaction fees, which are frequently well above reasonable if we're talking about tips. Perhaps a good Lightning Network solution is available for this, and you could offer it alongside BTC address. Or accept some popular altcoin with low fees (Dogecoin, perhaps?) along with Bitcoin.

Easily the only option that you should've done OP, that's what one of the bitcoin developers do, they put their addresses in their site's donation page, no fuzz. Also, why worry about Google ads? It's a good way to make your site a prominent one, in this day and age where there's billions of websites, you might want to consider being recognized, it's not enough that you're niche.

Just paste a BTC wallet address and some altcoin wallet addresses with a good Call To Action(CTA).
Just saying "Please Donate" won't do the work.The people have to feel inspired to support you.
You need to learn something about marketing and research some successful crowdfunding campaigns.
How do they gather donations?What is their marketing approach?
How do they communicate with their potential supporters?
Before thinking about donations,you should think about how to increase the amount of web traffic to your website.Are you going to use social media profiles,Reddit,Quora,Youtube or maybe some niche forums like Bitcointalk?
Do you want to SEO optimize your page,so it can appear higher on the search engines for a bunch of niche keywords?

Ok, so I made it super-basic. A QR code and a bitcoin address.

https://smokinghopium.io

Wen lambo?

Why would you want to use a third party for that? I think simply showing people your BTC address is more than enough. You could also implement a QR code for a simpler way to send tips. Most websites use a simple address text at the bottom of their web page and that seems to do the trick. However, I would be careful about hackers who try to hack into your page to change the address or by ddosing your web page and luring people unto a fake clone page while your page is down.