I prefer GoG and will generally purchase from there as no drm and you end up owning your game.

However their client is a bit shit and has been for a long time (GoG galaxy). You really only need it for updates though.

Steam excels for games with lots of mods/workshop. It just works.

If you want a decent product, use FoundryVTT. If you want something that needs a lot of work, use roll20

Why reinvent the wheel?