Just wondering, why not try swapping the battery? Worst case it breaks and you buy a new phone, best case you can keep your phone for a few months/years
I am still using the OP 6t I bought at launch. I will try to change the battery soon and move to Lineage OS and see if I can still use it for a few years
I installed LineageOS 20 a few days ago and I love it. I wanted to extend my OnePlus 6t lifetime but needed my bank apps so it is not de-googled (yet). I don’t know if it because of the custom ROM or just a factory reset but I am trying to run as many as possible foss apps and preparing for a de-googled experience