Odd results regarding charging after replacing battery

Hey so I recently bought a MacBook Pro 2018 off eBay, the seller had installed a new battery I assume as the cycle count was only 3, DSY manufacture from what i remember. When I first got the macbook the power button took a fair few tries to get it to turn on and it kept randomly turning off a couple seconds after the login screen but I installed iStats and caught it hitting 100c before it shut off, so I assume it was overheating, so I replaced the paste with PTM7950, all was well but it still kept randomly turning off, but once it was on it was fine, a couple days later the battery was jumping from 7% to 60% on charge and then going from 80% to 7% on the discharge. I contacted the eBay seller but it’s a store and they’re refusing to accept a return as I’ve opened it up, hey ho. So I got a OWC newer tech battery after seeing a lot of the 80% to 7% stuff around these forums attributed to a bad battery. So just installed the new battery, the randomly turning off appears to have stopped, still too early to tell. The power button still sometimes takes multiple attempts to turn on, but now Coconut battery is showing it as charging with 0 watts. Initially it’s charging at 20w (still seems too low?) then drops to 0 watts, but the battery percentage is going up regardless completely fine. The battery manufacture date shows 2015 but apparently this is an issue with coconut battery according to the company I bought the battery from. Any help would be massively appreciated, losing £500 on a macbook with a possibly bad logic board massively sucks lol