So I finally managed to progress to a point where I can do the chord changes I've been practising in a second or less, which I set as a kind of milestone.
Mistakes I've been making were the following:
- Looking at the fretboard
- Not practising enough with a metronome
- Pressing down too hard on the strings
- Horrible posture while practising
Besides for those things, I just needed more practice and callous build-up.
Thanks to all for the advice, it's been damned useful ?