Oh hey! A fellow M1P pilot! Glad I'm not the only one still dedicated to keeping this drone airborne.
I had that gimbal overload error a while back, and replacing the gimbal ribbon cable did the trick for me. It's a bit of a fiddly repair, but it's doable. I got my replacement cable off of eBay, but that was a few years back. It looks like the part's still available through a few stores for $10-15ish. I replaced mine almost 6 years ago and it's still going strong!
As far as the vision sensors, you should be able to calibrate using Assistant 2. It's not particularly difficult, you just connect your drone to your pc using a usb cord, select it in the Assistant app, then follow the prompts on screen.
If that doesn't fix the sensor issue, it looks like the bottom vision sensors should be pretty straightforward to replace. Remove the bottom heat plate (8 screws, steps 7 and 8 in this guide:
DJI Mavic Pro Front Arm Assembly Replacement) then two screws, a flat ribbon cable connector, and some silicone glue for each vision sensor. Here's an extra photo here that shows the rear bottom vision sensor in a bit better detail:
View attachment 176876
This store on AliExpress has decent reviews and has a two-pack of downward vision sensors for just over $3, if you're willing to risk buying on AliExpress!
https://www.aliexpress.us/item/3256801266248522.html?src=google&gatewayAdapt=glo2usa
EDIT: Just saw your reply about the sensors. Glad you got them working! The gimbal issue you describe sounds exactly like the issue I had before replacing the gimbal ribbon cable on mine.