MaM and Geek and some other sites post a weekly 0-Day dump from the comic usenet boards.
So every week, all the comics for the week a dropped in one bundle. “0-Day Week” is what they’re called.
Make an feed in your usenet/torrent/autobrr client to auto dl these weekly packs. Then in mylar add it to the folder-monitor.
To “quickly” (processing takes a while) jumpstart a collection go to MaM and download entire years worth of comics and have mylar import them all.
Please use MaM as having seeders is really important for all these comics.
I usually just use Tachiyomi and add sources that look like they might contain comics. Currently rereading Sandman.
What sources are you using?
Different person, but for western comics I'm using Read Comics Online, ReadComicOnline, and XOXO Comics. Haven't run into any issues finding what I need from at least one of these, but there are more options available too.