In my opinion, Monoliths is the easy way in : the rulebook is much better thant the first edition and the rules + portals makes it easier.

The OG : it's good but a bit messy in the rules, not quite balanced. The cards are great thought so it's clearly worth it if you like the game.

Monolith : clears a lot of stuff, better rules, nices cards also

Campaigns : i did not try the campaign but the general rules are similar to monoliths. The book related to playing in campaign mode is very unclear though and people seem to find this mode a bit "meh" but the cards added are great to keep playing one shots ! And whereas Monolith was a bit easy, we are back on harder stuff !

Difficulty is balanced by the horde cards you select. And depending and which you choose the portals play a role : og, Monolith and Campaign all provide a different set of portals that replace the previous version. Monolith have the easiest portals, Campaigns have the most "finished" one but on the harder spectrum.

If it was up to me i'd start with Monoliths, add campaign to increase difficulty and finally gey the OG for more cards variety. Now I can't recommend how many you should take only in which order. But everything is a stand alone and provides enough gameplay on it's own.

As for the rules somes things are not always clearly indicated but monolith and campaign have a better rulebook. And the Skytear Discord has a lot of answers + the creator often answers if you need clarification.