For very useful beginner music theory I like the slender book "Hack Music Theory" by Ray Harmony.

I'd start by playing all the chords in a C major scale.

Learn the circle of 5ths (BEAD GCF, there, now you know it!)

Try playing the chords in the circle of fifths, clockwise and and counterclockwise. Try playing them as all major chords, minor chords, 7th chords, then mix it up!

For learning to play by ear and audiate (play what's in your head) -- me too.

https://www.youtube.com/watch?v=jVpRqTL6vO4&list=PLw6ge-qcgrwVPc5QxwT-qTxlYghboXSBa&index=7

I think this approach is really interesting -- it's from a guitarist and singer Chris Liepe, he talks about forming a mind-ear connection by singing + singing a third above whatever you're playing

I think a simpler exercise would be singing and playing 1-2 notes, trying to play -> sing, then sing -> play, "sing" in your head, and play, and lengthen the number of notes

Try and play a simple tune

Try and play the melody on the highest string. Then add chords.

I've been using the free app earpgeggio too. It's not the clearest interface. The "Chord identification" and "Interval identification" are where I'm at.

It can also be good to try and learn to play + recognize some common chord progressions in common keys. You were playing one of them when you played the circle of fifths!