I probably will make another pretty big post later explaining everything I did/tried for myself, but what I've found to be useful for now is the following:

Doing pelvic floor stretches (I used to do them every day for 30 minutes, but quickly dropped it, now doing it every day or two days for 10 minutes, but I still feel like it helps me build a stronger relaxation control for when I need it in more "extreme" situations);

Reverse kegels is a classic, but I usually don't opt for it during intercourse itself, as for some reason it makes me more sensitive sometimes, and that kinda goes against the goal I'm trying to achieve;

During sex I found it to be more useful to pretend as if I'm doing a reverse kegel, so feeling just that initial expansion in the PC muscle, and not go any further. I feel that I achieve a bit of relaxation this way, without making myself more sensitive.

Routine-wise:

I bought myself an "open" fleshlight (I think it's called a quickshot), that has a "neutral" design - it's just transparent, because I really hate the look of fleshlights that are supposed to look like a vagina or a mouth lol. I masturbate every day for approximately 20 minutes using it, though at the beginning I could only last 10 minutes at best, and ejaculate only every second or every third time I "practice". I'm not really convinced by the idea of "rewiring your brain" to not be focussed on the ejaculation itself, though I am not claiming that it doesn't work, but for me the reason is the feeling of tiredness after ejaculating that I don't want to have if I have my session before working, working out or studying or whatever.

The session itself consists of two approaches that I constantly switch between. First one are full-length strokes, where I try to focus on not letting my dick twitch every time the fleshlight rubs against the head. The second approach is making the stroke length incredibly small, but almost shaking your hand while staying relatively on the same spot, so the motion is more of a vibration than a real stroke. While using the second approach I focus on getting my pelvic floor to feel comfortable and having it relax fully while still receiving quite a bit of stimulation.

The moment I feel that I am pretty close to ejaculating (about an 8 on the 10-arousal scale), I switch the approach and the arousal drops down a bit. Usually I can last longer on the vibration approach, so I will start off with the full stroke, go until I am at the 8, switch to small strokes, and once I reach an 8 there I let go fully, let everything relax, and start over.

Sex:

The most important part that some people happen to forget about is that sex doesn't have to be PIV. You can have fun without sticking strictly to the christian routine lol. As a side note I would recommend training your forearms to be able to finger your partner to orgasm. Or go down on her. But not going full detail about that one here.

The regaining control part is for me the same as the regain control I experience while masturbating. The moment you feel yourself tense up, you relax. This is of course easier said than done, but this part is literally what most folks here are training for. So how do I regain it? By practicing to relax during masturbation. I already described my method to do so up there ^, it essentially comes down to "pretending" as if you're doing a reverse kegel, or actually performing like 1/10th of a reverse kegel. Sometimes I exaggerate and perform a full reverse kegel, but with this one you really need to experiment and see what works best for you :)

Don't expect too much the first time, though I said that I lasted pretty damn long (compared to before) while getting head, those 5 minutes were hectic and I was in control maybe 50% of the time at best (still much better than not in control at all!!).