Acer Swift 7 (SF714-51T) full charge battery capacity keeps decreasing with every charge cycle

Hi there,

First time posting, so forgive me if there's some obvious solution which I haven't tried yet. 

So, a bit of context, I purchased a second-hand Acer Swift 7 laptop (SF714-51T), expecting there to be some degradation in the battery's full charge capacity which I had then calculated to be at 60%, much lower than expected. Now I could've returned the item for a full refund, but I had been looking for one of these units for a while and finally found one at an affordable price. The store I purchased it from provides a 12 month warranty, with an option to request repair, however the store's manager kept giving me the run-around and kept refusing to do this, even though it was explicitly stated in their warranty policy. 

Anyways as I really wanted to keep the laptop and hate contributing to e-waste, I managed to get a partial refund just in case I needed a new battery.

First I tried software troubleshooting, starting with a manual battery calibration (Charging to 100%, discharging to 0% and charging to 100% again), I'd repeated this about 20-25 times, however the full charge capacity kept jumping round all over the place between 60% and 5% of design capacity. I'd confirmed this with numerous battery reports generated by windows. I still kept getting unexpected shutdowns, incorrect battery estimates, etc.

Then, I reinstalled battery drivers numerous time, still no success in achieving a steady battery estimate nor a full charge capacity reading. Then I reset the operating system within windows, deleting all files. I had done this 3/4 times. Afterwards I'd completed a fresh clean installation of windows using windows media creation tool and booting from a USB drive, deleting all partitions when the option had allowed for it so the SSD would also be completely freed up. I still kept getting the same issue with full charge battery capacity going all over the place.

After doing all the software troubleshooting steps I could think of and search for, I then used a different high-quality charger but the problem remain unchanged. I purchased a new genuine battery (AP17A7J), which was pretty costly at £99.50. I replaced the battery and did a couple more charge cycles however, although it started at near the full design capacity, the full charge capacity started dropping with each charge, by around 300mWh each time which is around 2.5% of design capacity. 

I'm at the stage where I can't think of anything else other than some unobservable issue with a power-controlling component on the motherboard, which I've tried to inspect to the best of my ability but I can't see any damaged component. 

I know I'd kick myself if I don't try and at least exhaust all my options, where the final one is to ask the acer community, What should I do? Especially if the answer is a really simple and inexpensive one. Please help.