Hoping I can get some tips here as a beginner.

Been playing about 3 months. Got about 18 chords down, most of the fretboard, some finger picking & strumming patterns, chunks of a few songs.

Still struggling with barre chords, quick chord changes (espec. to barre chords) & thumb position.

Where should I go from here?

Horticulturalist. Environmental hand wringer. Open source enthusiast.  

Acoustic or electric? It's easier to learn on the latter, or at least a nylon string acoustic instead of a steel string IMO. Fast transitions between chords is something that only comes with a lot of practice and repetition, starting as slow as you need to play everything cleanly and ramping up tempo slowly over time.

Eng @ Coinbase, working on Base
https://bayardo.org

Acoustic, bought it second hand. Came with 3 nylon and 3 steel strings.

First knuckle of my index finger is aching from barre practice over the last few days.🤘

Probably should re-evaluate your technique a bit if your joints are aching! Key is to get to where you can barre the chord without having to grasp hard, which comes down to being able to place and rotate the finger(s) just right to get all the strings firmly against the frets.