Currently it seems that the app is really awesome but the 4MiB limit is causing problems for me, I have a Motorola Edge 2024 (XT2405-1) (avatrn) and its boot image is 14.9 MB (14,893,056 bytes) (Firmware dumps available at https://drive.proton.me/urls/F1FNMVRVV0#gs4Zw3w24VhC)
So using the app to modify the existing boot image is not possible