R20 is popular in the way 5e is, in my opinion, easy to use and generally not complicated. It's also been around longer then most of its current competition.
I haven't used FG classic in over a year so memory is a bit foggy but everything should always just auto save. Maybe you have folders set to read only or something? Also not sure how it works with PDF's, might need to change them to gif or jpeg. I've taken things right off Google search images, from incarnate and several other map making sites with no problems though. Also, always remember to update, both classic and unity patch just about every week. I'm still very much a novice myself but feel free to ask sometime and I'll see if I can help.
Foundry is the best VTT I have used and Pathbuilder 2e is the best character creator. I don't think you can import directly from the latter to the former, though I'm pretty sure they are working on that.
After several years of cultivating players, it has been a long time since I played with strangers. I've got no insight on finding them now.
I recently played a PFS game where a level 2 cleric tried to cast a second level spell.
My point is that characters created by new players who are unfamiliar with the rules should have their characters checked by someone who is familiar with the rules. Even a quick once-over. That is regardless of whether the character was created using tech or not. If absolutely nobody at the table knows the rules, then nobody is likely to notice mistakes, tech or no.
Amusingly, this particular example could have been prevented by Pathbuilder, which also lets you prepare and cast spells based on the appropriate slots.