To directly answer the OP's statement-question "Anyway, so from what I can conclude here, I should be able to use my current drop lever/shifter with any road bike mech disc brake calliper, is it?" No, the STI Tiagra shifter is not compatible with just any caliper. I know this from direct experimentation with Spyre calipers and Tiagra 10 speed brifters. It did not work well at all, in fact it is better to simply say they did not work. Not enough cable pull in the levers.
I also tried it with BB7 calipers and it did not work. Both the Spyre and BB7 calipers were MOUNTAIN calipers and require more cable pull. The BB7 caliper does come in a ROAD version and is intended to be compatible with road bike brake levers. I don't know for sure, but my guess is the Spyre calipers are available as road calipers as well, but find out for sure before ordering.
Again, I experimented with this myself and the results for both Spyre and BB7 mountain calipers were a failure when mated to Tiagra STI road bike brake/shift levers.