I suggest having Client States, not take up a diplo relations slot.

Why I suggest this is mainly due to a combination of other mods particularly Course of Empire, and client states unlocked (extended timeline) mods.
With this combination of mods, the only way to conquer the world is to segregate the world into client states.

Other than this. Everything seems compatible and I love the changes this mod makes!

You can create a march from your client state, marches don’t cost diplo slot and have more bonuses.

WOW... I actually like this change better than just a client state with no diplo slot.
Makes sense as you can turn it into a transitional state. Or if you permenantly (still can revoke march) want to keep it as a subject. You turn it into a March with no diplo slot. And that fact that you set a march at -9999.0% liberty desire.

I think this works perfectly!
My only suggestion now is perhaps making a page here with every detail of all changes made with this mod. Though it seems there is an extensive list of changes in your mod, so that's up to you.

I am also thinking about making the whole list of changes, but I don’t have enough free time to do it, so maybe one day.

fire of larestan decision give 1 colonist and this makes hard situation with extended timeline mode. Parthia starting colonizing whole world too early. I think colonist bonus should remove.

Yes, Parthia colonization can be a problem, however in all my Extended Timeline games, I was able to stop Parthia with country collapse mechanics - by just starting several long wars with it.

country collapse mechanics also have a problem. Rome collapsing too early and some countries like spain, france, holland revealing too early at before 100AC.

Extended Timepline is supported... bit mod still more focused on vanilla experience, so there can be such problems. Actually, as far as I remember, Rome is not stable in ET too...