I tried plucking the string upward and letting it loose
This is not done in standard slap technique – not with the thumb, that is. It's what normally the index finger does on the high strings, called pop, while the thumb slaps the low strings. It's of course possible to do that also with the thumb on the low strings, but it won't give you access to the typical fast, drum-pattern-like slap licks.
However, it's worth noting that the classic double bass slap technique, best known from rockabilly music, is mostly based on plucking up the low strings and smashing them onto the fingerboard, which is of course fretless. That way it is possible to get quite a lot of percussive treble, but this comes more from the fingerboard itself and isn't captured well by magnetic pickups.
Again, this technique does not use the thumb, but rather the side of the index finger, to pluck up the strings. And that's than paired with purely-percussive beats of the flat hand onto the strings+fretboard.
I tried bouncing on/striking it from above
That is indeed a usable technique on electric bass, but as you've noticed it's hard to produce much of a tone at all this way on fretless. It pretty much requires frets and fresh roundwound strings, and also enough practice in bouncing back the thumb real fast so it won't damp the string right away.
I tried striking through it with the left part of the bony part in the middle of thumb
That would perhaps be the “official” way to do slapping, and it can work even on fretless, but it's not easy. It requires slamming your finger into the string quite violently so it fits the fretboard. Yes, this will hurt for a while while you're learning. And again it's much easier to get the sound on a fretted bass, especially with low action. Also, ensure you're hitting the string quite close to the fretboard, but not on/over the fretboard itself.
Generally, uncoated roundwound strings give by far the slappiest results, but I also wouldn't recommend that on a fretless (with wooden fingerboard) because it's a recipe for wearing out the fingerboard very quickly. With flatwound, polished or plastic-coated strings however, it's hard to get any sort of slap at all out of the bass.
Finally, of course you won't get a good slap sound with any technique on any bass of the treble doesn't come through. Active basses make it easiest to bring out the percussion, but a passive one with tone pot turned up should be ok to. An amp with a HF horn brings out the slap loudest (in fact, perhaps too loud), but one that only has multiple 10" speakers should work good as well. Single 12" or 15" speaker, not so much.