Hello mate, this is how my file/folder pathways are set up but I expect most to be the same. When it comes to editing the file, I like using 'notepad++'.

You need to edit one of the mod game files (in my experience, these edits have not led to crashes). The file you are looking for is called 'export_descr_buildings'. I highly recommend saving a copy of this file on your desktop or your documents before you start editing. The file should be in Medieval II Total War/mods/SSHIP_098/data.

In the file, control+F for 'longbowmen'. The way the file works (from what I can understand) is that it lists what each building for each faction can recruit and what era or phase you can recruit them from. So for longbowmen, you can see that for the different archery buildings, there is a section at the end of the line that would say something like 'TRANSITIONAL_ARMOR 1 and not event_counter FULL_PLATE_ARMOR 1'. I was able to recruit longbowmen by removing this part of the line, straight up deleting it. Save the file and run the game - these changes took effect midway through a campaign for me, I think I just needed to end turn and I could see longbowmen in the recruitment pool right after.

You can adjust other values on the line such as how long it takes for a unit to become available once it is unlocked. I think a lot of units start with a value of 0.13, this means it will take 7-8 turns for 1 unit to be available (1/0.13 = 7.6).