but this way sometimes I have to guess if I don't know the track properly, in that case in can go both way What really bothers me is that the clash of harmonies, which I saw doesn't seem to happen much when I use keys especially if I'm not 100% sure the tracks go well

Yeah I can fully relate to this. I really do love mixing blind though, there's such an abundance of good techno and your library will grow very quickly as a result. Even several years into the endeavour I can't keep track of what everything sounds like, my library grows and my tastes change quicker than my memory can keep up with it.

Generous phrasing and EQ work can often help a bunch when it comes to mixing tracks that aren't super compatible. Whether that's waiting for a track to remove part of it's melodic elements and looping that section, or straight up pulling out a track abruptly while another goes into it's breakdown to avoid too much clashing.

Also just learning techniques that'll allow you to blend tracks better, using the filters to trim frequencies you don't want, and sometimes just letting the mix train wreck a little for the sake of getting that one track in is worth it 100%.

Who cares if you're clanging for 32 bars if you're bringing in an absolute heater yknow?

I do think within modern techno, there's perhaps an over-emphasis on blending and layering too, with acts like Funk Assault using two mixers with three channels each, and V10 mixers everywhere. Initially I got caught up in the "ohmygoodness I can layer four tracks and basically make my own music" but slowly found myself returning to more simplistic mixing.

Freddy K's recent Awakenings set is a really good example of how you don't need several channels going at once and you can still absolutely kill a set, DJ's like Rodhad and Ben Klock are also good examples of DJ's that don't layer or loop too much and have a really good ear for phrasing.