If you have already tried a new LCD then it would have to be the connection on the logic board. I don’t like swapping stuff until the problem is fixed, so I hate to tell you to keep replacing components, but you could try a new logic board.

Looks to me like the LCD is bad or some how the connector got loose. If you dropped it this could be the result of that, but if not it might just be faulty. iPads can be a pain to fix and diagnose, but if someone brought this to me I would replace the LCD panel. The screens on the iPad minis have connectors like an iPhone so it possibly could’ve come slightly disconnected, but you would have to open it up either way. If you do, just make sure you clean all of the adhesive off the housing before laying the screen back down.

It could be a software issue. You could try updating or just restoring the phone all together. If that doesn’t do anything I would replace the charging port and headphone jack ribbon cable. Here is a link: iPhone 6s Lightning Connector and Headphone Jack

Sounds like it could be a logic board issue. I would also check to make sure all the of screen connectors are properly inserted first. If they are fine and you have swapped the battery and it is still experiencing the same issues either the logic board could have a defect or the pads where the battery contacts the board isn’t getting a solid connection. If you have a spare board lying around it wouldn’t hurt to swap it. Issues like these are hard to diagnose because if you are like me you don’t want to blow $100+ on a part that might or might not fix the issue.

Make sure you got a good contact with the heat pipes after you reapplied the thermal paste. If that is not the issue than, the only other thing i can think of is the cpu fan isn’t working properly or it is a software issue. I don’t have a whole lot of experience with Manjaro so I wouldn’t be able to help you there.

It is probably the earpiece speaker that is going bad. Make sure the front facing camera and the proximity sensor are working. If they are both in working condition, I would replace the earpiece speaker first. If the camera or proximity sensor is not working, it is most likely the cable that contacts the earpiece speaker. Both of these parts are inexpensive and the repair is pretty straight forward. This is the link for the ifixit tear down of it, Remplacement du haut-parleur interne de l'iPhone 6s