In my current project I am most definitely using retopo (and several pieces of software). This is tedious, but rewarding in the sense that I am using much less system resources. Here is a very early example of what I have been doing. I’m much farther along now. I did decimate before and maintained detail, but the model was still very heavy.
there are some add-ons that can speed up the process, such as ‘contours’, but those cost money. however, you might wish to look into ‘b-surfaces’ add-on which is packed. it’s not as good as the ones you have to pay for, but it can save a lot of time.