The only drawback device tree (config.txt) doesn't seem to work e.g. gpu_mem=16, dtoverlay=pi3-disable-bt etc. Customized kernel needed or something?

I would think you need to be actually booting in device tree mode for that to work, not the EFI mode you used. It is one method or the other with these things.

the firmware can be found here: https://github.com/pftf/RPi3

also note: after installation, you will need to use the recovery mode to reinstall GRUB from the removable-media path, if installing to a USB (Which i HIGHLY recommend so as to not fry out the SD card where your UEFI BIOS is held)

https://git.devuan.org/devuan-sdk/arm-sdk.git

This hasn't been updated for a year and does not appear to work on Devuan Daedalus:

$ source sdk
-bash: typeset: -U: invalid option
typeset: usage: typeset [-aAfFgiIlnrtux] name[=value] ... or typeset -p [-aAfFilnrtux] [name ...]
-bash: typeset: -U: invalid option
typeset: usage: typeset [-aAfFgiIlnrtux] name[=value] ... or typeset -p [-aAfFilnrtux] [name ...]
-bash: typeset: -U: invalid option
typeset: usage: typeset [-aAfFgiIlnrtux] name[=value] ... or typeset -p [-aAfFilnrtux] [name ...]
-bash: typeset: -U: invalid option
typeset: usage: typeset [-aAfFgiIlnrtux] name[=value] ... or typeset -p [-aAfFilnrtux] [name ...]
-bash: zmodload: command not found
-bash: zmodload: command not found
-bash: zmodload: command not found
-bash: zmodload: command not found
-bash: autoload: command not found
-bash: colors: command not found
-bash: /home/ola/Sources/arm-sdk/lib/zuper/zuper: line 115: syntax error near unexpected token `say'
-bash: /home/ola/Sources/arm-sdk/lib/zuper/zuper: line 115: `function _message say act() {'
-bash: typeset: -h: invalid option
typeset: usage: typeset [-aAfFgiIlnrtux] name[=value] ... or typeset -p [-aAfFilnrtux] [name ...]
-bash: typeset: -U: invalid option
typeset: usage: typeset [-aAfFgiIlnrtux] name[=value] ... or typeset -p [-aAfFilnrtux] [name ...]
-bash: func: command not found
-bash: func: command not found
-bash: func: command not found
-bash: func: command not found
-bash: sdk: line 141: syntax error: unexpected end of file

All requirements installed, I think:

$ sudo apt-get install curl git wget qemu-user-static build-essential rsync gcc-arm-none-eabi gcc-multilib lib32z1 u-boot-tools device-tree-compiler lzop dosfstools vboot-utils vboot-kernel-utils libftdi-dev libfdt-dev swig libpython3-dev bc bison flex libssl-dev zsh sudo cgpt parted xz-utils

Any other options?

Edit: I'm looking for the equivalent of a mini.iso for the Pi 3A+, or if it's a complete system image one without any cruft (no Xorg, XFCE etc). Also: trust is an issue. Debian? Might as well go with Raspbian then. It's Systemd I want rid of.

https://wiki.debian.org/Arm64Port

Otherwise,

https://git.devuan.org/devuan-sdk/arm-sdk.git

Unzip it, then image write it to your card, (I use dd).

Yes, such a procedure works (i checked it before writing this post, specifically devuan_chimaera_4.0.0_arm64_rpi3.img.zip on pendrive) but such a system is already profiled in some sense. "from scratch" option as above is not available for this platform?

Unzip it, then image write it to your card, (I use dd).

In the SD-card-images directory, i do not see a device-specific file for rpi3 while the mini.iso applied to the flash drive, does not run raspberry - USB Mass Storage Boot set but nothing happens, no vision on the hdmi connector. It is possible to install devuan on rpi3 this way?

Regards,
technic-figure