Oriq Loremage is a tutor you should consider. Very good here, and very budget. Skirge Familiar is a good creature for dumping creatures into your graveyard and ramping for it.

It does appear that you could use some more card draw. I'd cut Selective Adaptation for something like Rishkar's Expertise. Minion Reflector seems ok, but I don't think it's what you want to do here. But, if it's an effect you'd like more of, Dual Nature might fit the bill.

@ bushido_man96 Thanks a lot for your comment (and upvote). I'm always happy about some advices and guidance :)

Oriq Loremage is already locked in.

I mainly want to use Minion Reflector to be able to distribute twice as much counters with Kathril, Aspect Warper to either my other creatures or Kathril itself. So while the original Kathril has to hold back with attacking (summoning sickness), the token (copy) of it can already swing in for some heavy commander damage due to the haste ability. To me this sounds like a good plan, or am i missing something here?

Carddraw-wise i feel the same - there should be more in it. But isn't Rishkar's Expertise a bit situational? What if i just got boardwiped? And if i have the "big guys" out already i am probably fine anyways even with a thin hand. Or is this just me, being an unexperienced mtg player thinking stupid stuff like this? Maybe just add "simpler" card draws like Night's Whisper etc.?

Here's the issue with Minion Reflector as you explain it: the "Legend Rule." So, you're going to cast Kathril, and then pay an additional to make a token copy of it. Problem is, since it's the same legendary creature, then you have to sacrifice one of them. If you sac the original Kathril so you can attack with the hasty one, then you not only up the Command tax for the next cast of Kathril, you also lose the token copy because you have to sacrifice it at the end of turn. If you sac the token copy, then you still didn't get to attack with it, spoiling the haste portion of it. It would probably be much better suited in a deck that is focusing on lots of creatures with ETB or LTB abilities. In here, I don't think it's going to do as much as you'd like.

You could probably replace it with one of those draw spells. Spells like Night's Whisper are good, reliable spells, and I think I run it my Kathril deck. But don't discount Rishkar's Expertise. It will do more than you think it will, for sure. Players are going to likely key on Kathril once it hits the board, and you don't want to run out of gas, so keeping the grip full is important. Return of the Wildspeaker is another good option. When Kathril comes down, he's probably looking at coming down with more than 5 power. Drawing 5 cards can be game-changing, especially in the late game when you are looking for answers.

As a general rule, I try to run at least 10 sources of ramp and 10 sources of card draw in my decks. If there is card draw on the Commander, I may cut that number down some, but not much. Kathril has a pretty high CMC, so ramping into him is important, so running a few more than 10 ramp spells might be a good idea.

Hope this helps!