lv_conf_template.h should not be used to create a lv_conf.h file as stated in the doc.
There already is a lv_micropython\lib\lv_bindings\lv_conf.h in the repo that was never there before.
Modifying this file with the settings that I wanted, everything now works.
Who would ever think that getting a garbage collection error would be caused by a LVGL config file.