Thank you so much Gordon for helping me. The reason DFU mode was failing was exactly the one you anticipated.
I was using arm-none-eabi-gcc version
8-2019-q3-update to compile the firmware and indeed at the end of
bootloader...hex was this line
:08E00C0011C30700C1C10700A8. After switching to the same compiler version as yours I got this line
I tested an was able to perform BLE DFU upgrade from version
Thank you again for your reactivity and your willingness to help.
Finding a way to get a few more bytes out of the bootloader is definitely out of my reach. But I wish to contribute by filing bug reports and sharing small apps with the community.