I spent a month making a thorough garrison mod with 3 tables for my own game. You have about 12. This will cause complexity and incompatibility issues at first look.
Why don't I want my units to upgrade? I want my units to be the latest version of skirms and spearmen and also want them to reflect the local armour workshop.
Why is the purpose of doubles other than the separate icons? The icons are pointless if the landunits are the same? Am I correct that the landunits are the same as the campaign landunits?
Puzzled by need for "uiunitgroupsings, unitsettounitjunctions"

One problem I had was that it was impossible to untangle the garrisons for the barbs. They use each others' units. Did this fix this? Why not just use unitogroupingsmilitarypermissions for that?

As I said, "I made it really simple, actually. They are doubles: copy-units. By copying them I could make sure that they don't upgrade like the regular army units and can have their own category icons which make garrison units easy to recognise in battles. They will have a slower replenishment also and by doing this the mod will be more compatible with other unit mods."
If you study the tables you will see that there is quite a logic in it... and really easy to modify to your liking. A lot easier than the vanilla tables. I split the tables up for a reason. I like garrison units to be without upgrades, that's a personal thing. All simple units apart from some decent melee units (as explained earlier), they don't need to be super troops, you can garrison a regular army if you want to supplement it.

As you say....compatibility issues etc at first look... Look closer. I know what I'm doing and everything is done intentionally. The 12 tables are mostly for the copy units and the category icons... They will not interfere with any mod I know of, because nobody copied these units and nobody ever added category icons. I personally like to know which ones are my garrison units... because they have disadvantages (staying lower tier, slow replenishment) & advantages (they are free etc.) compared to regular troops.

The untangling is done by making them copy units... you can assign those to the factions you want without interfering with the vanilla units. I use unitogroupingsmilitarypermissions for that... But it's different from the vanilla units, because I don't want to take any vanilla units away from the unit rosters of the factions. E.g. ostrogoths... They have many Roman units apart from barbarian.

-uiunitgroupsings: for category icons... It's a double of the vanilla, but for the copied garrison units only.
-unitsettounitjunctions: I added some new units based on vanilla to keep the rosters similar...

"can have their own category icons which make garrison units easy to recognise in battles. "
"I personally like to know which ones are my garrison units... because they have disadvantages"
I am not seeing this in battles. What should I be looking for?
Is this an unfinished work? What can I add to make sure the battle icons signify garrisons?

I am still working on a version of this and would like to improve it while I am immersed in it. All my other garrison mods have had trouble with consistency due to tech upgrades.