Sounds like the best fix to this would be some sort of soft cap for clans in a kingdom (based on amount of fiefs) after which the AI would start considering expelling landless clans.Unfortunately knowing the problems does not mean they will be 100% fixed. I am trying to underline this problem for long time (started 2-3 years ago) however situation is still same. So I cannot give you a guarentee for fix. For any problem, (1) possible solutions need to be found and (2) these solutions need to be approved then (3) lastly they need to be applied. However step-2 is a bit hard to pass here.
For example currently a new feature rebellions is added to game. According to design after a rebellion occur a new clan is added to game. Yesterday I made a new test run and collect datas to see if everything is ok after rebellions is implemented and see that total clan count increases 1.5x in first 20 years so lord party counts too (actually lord party count become 2x more than clan count increase because all clans also tier up and spawn more parties by time). I prepaired charts / graphs / data tables and reported problem. Offered 2 different solutions for fixing this problem (npc clans to tier up 50% slower (yes it is a cheat not perfect solution but we need to slow down tier up process for npc clans to lower number of parties at late game, fast tiering up of npc clans is not important thing for gameplay it is only important for player to feel progress) & rebel clans to be removed from game if they cannot hold their initial settlements (most cannot hold)). These 2 solutions were solving problem mostly without damaging other mechanics. However they are not accepted and no new solution is offered yet. So this problem is left in game. So currently number of clans / lord parties at game consequently increases after rebellion feature. All efforts done previously for balancing game is damaged with this addition because now we have 2x party on map at late game (20-30 years after starting) it was 1.3x previously (from tier ups). So settlement per clan decreases by half by time, this decreases tax income average per clan, this increases ratio of poor lords so parties start to recruit less people to their parties, when all lords are poor and has less men they stop wars and all world go into peace. Also because of income decrease kingdoms stopped hiring mercenies at late game. As summary lots of balance and game mechanic get hurt after one new feature.
I am trying to give an example. As a dev we are not in so different situation with players. You play the game and see problems and you offer different solutions to your valid problems and when you see there is no development about your problem for long time you can feel that your possible solutions are not cared. However it is similar for us too and knowing the problem sometimes does not mean it will be fixed. We prepare tables offer solutions and sometimes nothing is done.
After that you could make those clans into mercenary clans and remove them from the game if they fail to keep themselves afloat as mercenaries or have other kingdoms take them in if they need more clans.