There's not many good options to be honest. The best installers, DBI and Tinfoil, are both slightly shady/closed source. The next best option is Awoo and Tinleaf, but those don't have MTP Responder and can't properly install block-compressed nszs.
It's become pretty strict due to legal concerns, but there was a shop that was giving out access to new accounts during their anniversary event recently so that could be something to look out for. I think Neko's homebrew store is still available to add onto Tinfoil but you likely won't find official games there.
If you're lucky enough to have some missing DLC though, then definitely feel welcome to contribute to preservation efforts.
There are sites where you can download the game files and install via tools like Goldleaf or Tinfoil. Both seem to support installation over USB cable, so you just have to connect it to your PC and run the tool to install things.
Possible, but you'll need to use torrents, DDL sites, or usenet. rutracker.org has lots of switch roms, and 1337x.to have some as well.
For shitty ddl options, we've got nsw2u.com and nxbrew, but you need to make sure you have uBlock Origin and FastForward extensions. JDownloader2 can also help, since you can grab the shortlinks to the rom files, plug them into JD2, and JD2 will automatically grab the file and bypass the redirects.
You could grab nszs/nsps at the above locations and then use DBI MTP to install the nsp directly to your switch from a PC.
If you can donate a missing dump to get access to Pro Shops, you will also get access to Neko Drive, which is another great source of games, served directly from Google Drive, no Tinfoil shops or blawar software needed.