I looked at their Euphorbia list. I've grown 3 of the plants they offer (horrida, ingens, grandicornis) from seed and those were all pretty straightforward with good results. I'm not sure one would necessarily want to grow those plants from seed though, given how well they grow from branches/offsets. All three plants tend to get pretty big after a few years, like not so portable.
You can learn some things by watching baby seedlings (ingens and grandicornis self-pollinate, but horrida generally does not and can be variable depending on the parents). Like we talked about earlier relating to differences in how seedlings behave vs. cuttings in the case of ingens. For example grandicornis does a weird thing where the main stem branches very early, and there ends up not really being a main stem afterwards as a result.
My cactus experience is mostly limited to natives. Outside that group, in terms of aesthetics, I like the white hairy plants, for example these 3 types
but I don't think they would really be a good idea where light is limiting. I see they have O. celsianus (a nice big one) which is another plant than can self. My germination rate was low but the baby plants came out strong.