Thanks so much for this wonderful comment! :D
Firstly, yes FE was 100% the inspiration. There's a little bit of Shining Force in there too, but mostly FE haha.
Units not being able to attack on their starting tile is indeed a bug, thank you for catching this one! I rewrote the pathfinding system from the ground up in the last 12 hours rather sleeplessly so I can see how this one slipped through! Hope to adress this in a post-jam update.
In terms of acheiving this in GMS: data structures, data structures and more data structures. You'll be relying on ds_grids a lot in particular. This is good because you can plan out a lot of this stuff in spreadsheets and visualize your code better. Setting up a backend of scripts that can easily add/remove/edit things in your inventory or map is a must. The first few days of the jam I didn't have much gameplay to show because all my time was spent just setting up these systems! A good dynamic menu system is also pretty important in an RPG like this as it's how the player spends most of their time interacting with the game.
Also if you are interested in the isometric stuff, this is a great article that helped me immensely: