If you have a 2012 you can easily replace the battery yourself, just check the relevant ifixit guide for the screw bits you need, IIRC from my 2010's replacement the battery has a pair of screws (one of which I think is behind the HDD bracket, though I may be misremembering) and an adhesive strip on the back which can easily be overpowered (though it's probably a better idea to soften it using heat from the trackpad side).
According to coconut I replaced my battery back in 2017~2018 or so (battery was manufactured in October 2017) and it was a breeze. "New" battery is at 6934/7000mAh (99% capacitity).
I think there are early and mid 2012s that differ quite a bit. I have a mid 2012 and the battery replacement is a nightmare. I'm waiting for my 14" to arrive, I'll switch over, then give it a go so I can hand me down the 2012 with some life in it. And if I blow it so be it.
I have this computer too and I recommend the replacement sold on BH photo video. I just put it in and I can somehow get like 7 hours plus of use from this computer doing very light stuff (like reading hn). Impressive for a computer so old. Still very performant for me imo with 16gb of ram and an SSD upgrade under the hood. I was looking at the m1 but I'll hold off, nothing pushing me away from this device currently and it seems like I will have software compatibility issues on ARM until they refine rosetta or offer bootcamp again.
Replaced the Retina 2012 15" MBP battery fairly recently, as well as the SSD a long while ago (with an adapter) and took maybe 8 minutes, minus the screws, each time. The worry is the 3rd party batteries won't last long or will swell sooner than OEM parts.
According to coconut I replaced my battery back in 2017~2018 or so (battery was manufactured in October 2017) and it was a breeze. "New" battery is at 6934/7000mAh (99% capacitity).