LVGL9 shouldn’t be used yet because it has a changed API. So you’re at a good path by switching back to 8.3 branch of LVGL. From your error log it seems your Arduino build still wants to get a source (lv_windows.c) which is only present in LVGL9 and not in LVGL8 (where drivers are still at a different external ‘lv_drivers’ folder).
Arduino IDE does a lot of things in the background that you can’t see but hopefully it helps if you completely exit Arduino IDE, and start from a totally new export, this time selecting LVGL 8.3 everywhere where asked, and hopefully this way there won’t be residuals of LVGL9 build-scripts/configs.
Btw there are more Sunton-specific board-templates spread around here at the forum made by other users, I think you should try them if they’re better suited than the general-purpose Arduino TFT_eSPI board-template currently available in SquareLine Studio, kgschlosser’s is what first comes into my mind: