Ipad Battery life still poor after battery replacement

Hello everybody,

I am facing a really weird and utterly frustrating issue. I use my ipad pro as a notebook in which i take all my uni notes. As it turned out the ipad could only last around 4 hours on-screen time(I would usually just take some notes in notability with my apple pencil). Since I got the Ipad second hand with no warranty left I was forced to replace the battery. I had it done in a 3rd party Apple service. From the beginning it did not feel any better- except for the fact that a battery app showed “perfect condition” as opposed to “15% of wear” previously. Ipad does not show apps draining power in settings/battery - it only says to use the ipad for a couple of minutes but it does not change at all... at this point I do not really know what to do. I will definitely go to that service again, but I have 2 theories on what is actually going on:

1. Ipad won’t fully work with a non-original battery

2. The U2 module on the motherboard is broken. (Or actually any other module).

Also:

I have tried restoring the device- didn’t help at all.

I have played a 10h long video on youtube and the ipad went down to 37% having played only 2h and 50min of the video. (The video is still playing as I am writing this post).

I would really appreciate any help as I feel really let down by this iPad. I use it daily but this battery life is far from acceptable in my daily routine.