I think the Anti-dupe system you proposed isn’t good enough, you can still get alternating dupes, instead make it so it picks from a pool of weapons and when you get a weapon it’s adds 1 of every other weapon to the pool and reset a weapons quantity to 1 whenever you get that specific one.
Then to help make the weapon bonus rng not painful, when you spawn a Lich make the % change of getting a weapon eg 1.5x when you have the regular variant equipped and when you have the Kuva variant make it a 2.5x multiplier. You could also make the base level of the node influence the % value so a sortie 3 Larvling would have a 50-60% bonus for example.
With these 2 features the rng would be less painful, and when you have everything you want you can slightly target your farm and try get your weapon to a better %.
Also a way to just trade liches without having to convert them first would be nice even if you increased the trade tax for that.
Another QOL would be to let us for a cost eg 1 million credits or a challenge eg lvl500 mission get rid of a Lich without having convert/vanquish them even if you put a limit of 1 discard per day or something like that.
As even with the discard you still have 13 weapons to farm and get to 60% weapon bonus (or if it’s a weapon with base elementals a different % for however you want to biase Status procs) and for all the elements you want to have so that is 13 * 40(different bonus %) thats 520 possibilities even at 2 a day that will take almost a year then consider you may want multiple copies of some weapons with different elements so that would bring you to over 1-2k possibilities. Even if you would be fine with 40-60% bonuses that would only bring that down to 500-1k. Seeing as for warframe this is some silly rng considering the time it would take to farm them all the some changes to reduce rng would be VERY wellcome.
CONCLUSION - the rng reduction needs improvement so that getting all the weapons you want and getting them above average % bonus would take a few months instead of years. (Especially if you are planning to add more Lich (or amalgam *wink*) weapons down the line)