MacBook Pro A1707 trackpad shaking

I've got this old Macbook Pro that was with a very swollen battery. Decided to replace the battery, ordered a new one, then found out that the trackpad and keyboard flex cables have broken due to the battery being so swollen.
So I ordered new cables as well. After putting everything together the laptop works, but the cursor is shaking and unresponsive. Unusable.
I've removed the trackpad dozen more times to clean it and check if everything's as it was. No joy. Then accidently cracked the trackpad, so I ordered a NEW one with a new flex cable. Doh, still no joy. The cursor acts identically. Putting external mouse works, but sometimes the cursor still barely jumps in random direction.

At this point I'm all out of ideas, so any advice would be helpful.