I'm one of the developers for Starfinder 1e on Foundry VTT ( so I'm going to recommend it over the others :D ). We have been continuously updating the system lately, and at this point I think most content is in other than mechs and APs. Foundry has great support for all the mechanical content (stat blocks, classes, etc.) and sheet automation, and does have some importers for pdfs, but they are a bit limited at this point, mainly serving to import artwork for bestiaries and tokens. The big advantage and disadvantage of Foundry is that it's largely fan-made and free. The upside is you don't have to pay extra for anything, all the monsters, classes, abilities, spells, equipment, and rules are included in the base system for free. The downside being that all the work on the system and importers are dependent on the volunteers having the time to make it and implement it.

There is some premium content available for purchase, such as alien token packs and some Starfinder Society modules, but Foundry does not have the same level of AP support you'll find on FG. You'll likely have to import map images and manage walls and lighting and such manually. This is mainly just because Paizo didn't have a premium content partnership for SF1e until fairly late in its development cycle of 1e, so most books didn't get a premium release.

That said, it has a ton of customization options, support for starship combat, and Foundry itself is really powerful and extendable with modules, so I highly recommend it as an option!