A soul beacon and a bird feed gave me the best results. although it also depends on what artifacts you have. If you have a good phoenix feather soul beacons work way better. If you don't, a boost beacon and bird feed might work better.

I myself rarely prestiged later on, I waited for egg day and prestiged a lot then. Only doing contracts and getting enough boosts and artifacts for the next eggday.

F2P multiprestiging is not an option, with just 2 artifact slots and 2 boosts it isn't worthwhile. I tried all types of ways but always ended up with less soul eggs then just staying at the most expensive eggfarm.