Depends. Do you want more battery life?

I don’t think any Nitro laptop ever has gotten good battery life. So as long as it’s not 100% toast I’d just leave it plugged in 24/7 like probably most nitro users do.

I would say its worth it to get the battery for now. Hard to say where the PC market is going and honestly things don't look great, but if its still working then I would say the battery should hold you over in case things do get better for the PC market.

While you're in there you can also clean it out and reapply thermal paste so that it helps the CPU and GPU (if it has one) last a bit longer as well. Might even boost performance a bit.

The battery might swell as it ages. But it probably won't. Then the question is: do you want or need more battery life? Is it worth the $50 or so that a new battery costs? If yes, then do it. If it's always just plugged in anyways, then maybe pass.

Do you need the battery or do you always run it off the lead?

My laptop battery is at 66% after only three years, but it also charges and discharges a lot because it's my daily driver for university. I've also thought about replacing it soon, a few years I bookmarked the replacement battery on the company website in advance. My plan is to keep this laptop for eight years.

wait until there is a problem to solve it

If you hadn't upgraded it I would say it'd definitely be better to keep on chugging until you can afford a replacement, but investing money into parts for what you have kinda changes it

I know you're attached to it, but maybe look into trade-in value or how much you could sell it for? That might close the gap a little on an upgrade or at least equivalent but new replacement, especially so if the newer parts increase its value enough