But how can you sell from non-custodian wallets? Most of my coins are in hardware wallets. But I was planning to sell some btc to take advantage of this bull run but because of this crap Coinbase I can't

Is there any other way to buy/sell without using exchange?
This is the way I do it, I make use of noncustodial wallet, and I can make use of noncustodial exchange, you just convert to btc to stable coin for that time period, and convert back anytime you want. Or, you can follow this best way, making use of Dex which would be fine, but the fee can be too high like $6 for exchanging from one coin to another but still better so far you do not trade often. There are some noncustodial wallet that have third party Dex support, you can just swap directly using the Dexs on the noncustodial wallet.
But, I would still advice you not to convert to any coin, the possibilities that bitcoin will reach all time high is over 99.99%, you can just hold your bitcoin than converting to shitcoins.