I had to get a new battery for my OP6T after using it for over 5 years - exclusively charging it using the fast charger. I'm sure that I can get another 3 years out of this phone, at the very least.
The battery + replacement service fee was around 40 USD.
I don't expect much more from the battery TBH.
Bro, which company expected you to do a take home assignment that took 2.5 months? Name and shame the fuckers.