Is it realistic other clans want to leave their kingdom and join you when you have no fief and less power and they already have fiefs in their kingdom?
Actually I believe it is. If the player has been winning many battles and has great relationship with the clan and the clan has plenty of money in the bank, the noble can expect to gain more fiefs in the long run by siding with the player.
Historically, noble families would often side with outsiders who they thought would win or who they believed had a better claim or who they thought would be a better ruler or who they thought would reward them more. They also knew that those who joined later - when the fight was decided - would be rewarded less.
To be fair, this was also usually in the context of civil war rather than invasion from outside, but - unless we have a civil war mechanic - then that's not an option. Maybe an additional feature in the future would be having a choice between getting the clan to defect with no fiefs vs getting the fiefs but starting a war.
Of course maybe with future additions denars opponent clan want to change sides would change according to persuasion steps however currently that system is not working like that so best is breaking dialog at first if final amount is too much for player.
I understand that the system is limited currently.
I don't remember exactly how it worked in Warband, but I remember nobles used to bring up the specific reasons why they were hesitant to defect and having the player face difficulty convincing them on each point. If there are very good reasons why the clan shouldn't defect, then that should be handled by making it more difficult to convince them - not by making it more expensive +to buy their loyalty.
However the "make a kingdom " quest misleads players into thinking they will do well as a kingdom in a grossly underpowered form. I think this should be replaced with a more thorough quest line with many steps that actually teaches a new player how to build relations, save money, what power means in the war score and other so things they can enjoy becoming a kingdom and not rage quit (or take evil lessons from me on how to overpower the AI alone). It would probably be better if it wasn't the 'dragon banner' quest with it's weird side quests, but just guide to prepare to make a kingdom and get vassals.
YES! Having a reason to interact with nobles was an important part of Warband and - while the mechanics were difficult - it was a much better system than simply spending influence to get them in your army.
If having the dragon banner of Calradia and earning enough renown to get you to clan tier 5 in a handful of years and single-handedly starting a new kingdom isn't a major factor in getting someone to join you... then the game should add additional steps along the way to make it work better.
EDIT: OH! Maybe it should be easier to convince lower clan tiers to join you rather than high clan tiers? That way the player is rewarded for grinding renown and will still be able to have backup in the relatively early kingdom-building stage.
One thing I was very frustrated by was when I married my sister off to a Khuzait clan and got 100 relation... then saw that the clan chief hated Monchug... then released clan members in battle... but they still wouldn't consider joining my faction without millions of denars.
Having high relation in Warband meant you had an ally who would might leave their faction to join you and would agree to help you with big fights and would be loyal to your cause. I don't know what it means in Bannerlord? Just cheaper influence costs? It's a weak and one-dimensional mechanic.
The most likely lords to defect visiting the player's court - or sending a messenger to the player letting them know that they're ready to talk and where they currently are, if TW doesn't want to complicate the AI further by introducing court visits - would be a great way of dealing with the problem of lords being so annoying to hire.
YES! One of the only reasons I kept playing was because Diplomacy Fixes messenger system made life so much easier in this way. I'd have given up on the game entirely if convincing a clan to join me meant that I had to chase each clan leader down individually when they're not in an army and pay millions etc etc etc
Having the clans send messengers to you would be so much better. Especially if - for instance - you're besieging their fief and they want to offer you money to go away OR offer to join your faction so they keep the fief.