It has been a very long time since I tried GIMP (>15 years) to remember everything I found wanting, but as I recall, GIMP lacks both macros and batch editing, the former letting you record a set of actions to a hotkey so you don't have to repeat them yourself all the time, and the latter letting you apply a set of actions to hundreds or thousands of images at once. I would literally have to spend hundreds of hours to do things in GIMP that can be done with no effort in Photoshop, to the point where it would actually be easier to just program something myself from scratch than it would be to use GIMP, if Photoshop didn't exist.
I see that GIMP has since gotten a UI revamp, but the multiple window UI from the time I used it was also unbearably bad and one of the main things that sticks out in my memory.
Have you looked into script-fu? It would probably be a very steep learning curve.. BUT there is an opportunity to do something impossible 10 years ago, and that is to use AI and an external application. BATCH-FU is one such attempt but it seems to be a 'select action from a menu' thing.
But Gimp developers: implementing batch in one go is a big ask I know. But a great first step might be to create a channel in Gimp where correct script-fu is emitted for operations in progress. Being able to connect to that from outside would allow 3rd party projects to assemble "record by doing" macros that could be turned into Photoshop-like batch capability.
Macros are on the roadmap (https://developer.gimp.org/core/roadmap/#macros-script-recor...), and in fact we did a lot of prepwork for them during 3.0's development (internally, several features like filters and plug-ins now have configs that store settings, which will be used by macros in the future to repeat operations).
I see that GIMP has since gotten a UI revamp, but the multiple window UI from the time I used it was also unbearably bad and one of the main things that sticks out in my memory.