The best way is buying campaign/investigator expansions. Note that both are necessary for a complete cycle. Fantasy Flight have also starting reprinting older (and harder to find) sets in the campaign/investigator expansions; purchasing both these sets is equivalent to buying all mythos packs for a cycle. If you search „Ermittler-Erweiterung" or „Kampagnen-Erweiterung" for your preferred cycle, you shouldn't have an issue finding a set.
The „Return to-" sets are further expansions to a cycle and intended to give a campaign greater replayability. I would not recommend them instead of the original campaigns.
A note on card sets. While your current 1st Edition Core is still useable, it is a 2-player box. There is a revised version which accommodates 4 players (and includes some cards from expansions as part of the core) though if you are solo or want to play with just one friend, then the original core set will work.
Edit: While I don't endorse this method, I have played with a friend who grabbed high-quality card images from an Arkham Horror module for Tabletop Simulator and then printed out playable sets on cardstock.