I think the main problem are the systems where you first farm the "lots" and then you run the missions to get your expected reward.
Becourse it feels realy unrewarding if you don't get what you worked for and all your "lots" are gone (worked for nothing). In the "prime part" system the problem is reduced via trading. But still, I would prefer a ducat market for prime parts.
The Kuva Lich System is not so different from the "prime part" system:
1. you need your relicts
2. you get your parts (mods)
3. find out the mod comb, kill the lich, get your weapon.
It only adds with 3. another grind layer, and thats not the real problem, becourse it only adds time, which can reduced via luck, but you still progress with every mission. And I actully think that this is a good kind of rnd in the game, where you can be lucky, but if you don't it doesn't upset you.
The real Problem is that you can do 1-3 and still it's total rnd if your weapon is any good, becourse of the elemental bonus. To get rid of this rnd based bonus I would prefer to get some kind of "kuva heart" as a fix drop from every lich, which you can use to upgrade the elemental bonus of your kuva weapon. Maybe starting with 25%->35%->45%->55%->65%. Or increase it with every forma. To lower the grind on the mods, I would do a trade in system 4 to 1 or 5 to 1, where you can choose your Requiem Mod.
Of course, now you can trade lichs, but the prices are high, becourse you can only trade the hole thing and good weapons are rear.
For Rivens it would be nice if there states depend on the amount of kuva spend on them. For example:
0 - 10k 1 low stat
10k+ - 25k 2 medium stats,
25k+ - 50k 2 high stats,
50k+ - 100k 3 medium stats,
100k+ 3 high stats or 3 very high with 1 neg.
Still got a high rnd faktor, but if the "stat level" of low/medium/high got a relativ small range it would feel realy rewording to sink a lot of kuva in a riven mod.
Over all I would perfer, less rnd in the game and more constant progress and I don't understand the elemental bonus on kuva weapons, a range of 25%->80% TOTAL RNG REALY.