After month of off/on playing,
I'm having extrem difficultying earning plex for even the basic schematics from shop :(.

Is there a cheat for plex?
I want to try multiplayer, but I don't want to have limited options.

Try some single missions. Zombie mode in particular is a gold mine with the correct ship. If you cant afford to equip your ship, use the pre-built designs. Also, if you are playing for a month you should have enough cash from paid research...

You can also play some difficulties on a harder level which adds a multiplier to the plex earned...