Hi, these are the steps you need to follow to edit the startpos in order to change the number of building slots. I am going to use the Radious compatibility submod as a reference because it contains the changes necessary for this task:
1- Using RPFM, extract the update_commands_radious_compatibility.txt from the Radious compatibility submod.
2- Launch the Assembly Kit in Steam and select DAVE. Go to Local changes->Apply Commands from File. Select update_commands_radious_compatibility.txt, this should generate the required startpos changes.
3- Open the start_pos_regions table and modify the slot_cap column, which determines the total number of slots.
4- Export -> Export Changes to Binary. Close DAVE.
5- Make sure that there are no mods active. Launch the game, unselect all mods, and launch the game, then close it.
6- Launch the Assembly Kit and select BOB. In the working data column, tick campaigns. This will open a window, tick the phar_combi campaign and the "Create plack file" option. Click Start button. This will create a file called mod.pack in the Total War PHARAOH DYNASTIES\assembly_kit\retail\data directory.
7- Create a new mod in Total War PHARAOH DYNASTIES\data directory using RPFM. This will be your submod. Import the start_pos file you have just generated (the one in mod.pack).
8- Import the campaign_building_chain_slot_unlocks_tables table from Radious compatibility submod and edit the Active Slot Count column. If you want to have 10 building slot settlements, the max number in this column (for level 5 settlements) should be 9.
9- Import phar_ancient_legacy_khufu.lua from Radious compatibility submod. Replace all "slot_number = 12" with "slot_number = 10".
9- Optional: Upload the mod to the workshop. Mark Expanded Family Trees as a required mod.
10- Load your submod on top of Expanded Family Trees. The Amazon mod should go on top of both.

Oh wow, thank you very much. I'll give this a try later as I do not currently have the time, and follow up with any subsequent questions if I have any.

Quick question though, is using the AK easier/better than just using RPFM by itself? Or is that not even possible.

The DAVE tool in the AK is a table editor, so it's an alternative tool to RPFM, which also allows you to edit start_pos tables. However, opening and editing tables is very slow, plus it's quite restrictive when it comes to editing fields (the field you are editing should have the corresponding entry in the table it points to). As a rule of thumb, only use the AK to edit start_pos tables. Once you have created the start_pos file, edit the db tables with the RPFM.

Yeah sure, happy to help. I'll be travelling over the next week, so I'll not be answering as often as I do.