Caveat, IANAE
So if you remove a bolt with threadlock on is it best practice to replace the threadlock with new?
I do
If so do you need to clean off all the old stuff and in that case how do you get the stuff off the inside of wherever the bolt screws into?
I clean it to remove any grease and loose bits
Finally, if you do replace it/add more on top, how do you tell what strength the original stuff was?
I work off the torque setting, size and material of the fastener. Compare this to the breakaway torque in the manufacturer’s spec, 6Nm torque setting gets 6Nm breakaway threadlock
EDIT: Breakaway spec varies with the size of the fastener; a larger fastener will have more surface area and will need more torque exerted to breakaway than a smaller one
Some proper info here https://www.loctiteproducts.com/en/know-how/build-things/threadlocker-red-or-blue-which-ones-right-for-you.html