OK
Back to 4.3.3-1-osmc then
quiet remove loglevel=7
Reboot and black screen with only one line : Welcome to Open Source media Center!
I stay on that state before going to grab some logs just in case you want to test something else before. Tell me.
EDIT:
I forgot to mention that of course I change the dtoverlay too to get that at the end
dtoverlay=lirc-rpi:gpio_out_pin=17,gpio_in_pin=18 disable_overscan=1 over_voltage=0 over_voltage_sdram=0 force_turbo=0 disable_splash=1 core_freq=450 initial_turbo=0 start_x=1 max_usb_current=1 sdtv_aspect=1 gpu_mem_1024=256 arm_freq=900 sdram_freq=450 hdmi_ignore_cec_init=1 dtoverlay=mmc
M.
PS: to let you know, my mini keyboard has no Function keys so I can’t say if the ALT+F2 is working or not.