Hello Alexandre & IOhannes,

1- Thanks a lot for your guidance and help. 😎
ELSE 1.0-0 rc-9 Pre/Test (downloaded today from Git) is running well under Pd 0.54.0 / RPi OS-11 32-bit (armv7) base Debian 11 / RPi 400 (h/w).
I tested it with one of my MIDI patch using, among other externals' objects, [else/midi], [else/keyboard] and [else/sfont~].

2- For people interested, I recap what I did on my RPi 400.
- Pd 0.51.4, 1st install from RPi repository (base on Debian 11 32-bit).
- Then, successfully updated to Pd 0.53.0, 0.53.1 and 0.54.0 by compiling them from their source code downloaded from Miller's site and then installing them.
- Latest ELSE external accessible thru Pd/Deken was v.1.0-0 rc-6.

3- As Else v.1.0-0 rc-9 for ARM is neither available thru Pd/Deken nor directly from Alexandre's Git, I decided to try to compil it.
My 3rd and last trial was successful and it took about 10 minutes.
- Download pd-else-master.zip and uncompress it where you want, then follow Alexandre's Building instructions.
- With my RPi 400 setup this gives in a Terminal:
. 1st pass in 2 steps:
    pi@RPi400:~/Downloads/pd-else-master $ make pdincludepath=/home/pi/Desktop/Pd-Compil_RPi400/pd-0.54-0/src
    pi@RPi400:~/Downloads/pd-else-master $ make install objectsdir=~/else-build
. 2nd pass also in 2 steps, but only if needed for [sfont~], [sfz~] and [plaits~]
    pi@RPi400:~/Downloads/pd-else-master $ make sfont sfz plaits pdincludepath=/home/pi/Desktop/Pd-Compil_RPi400/pd-0.54-0/src
    pi@RPi400:~/Downloads/pd-else-master $ make sfont-install sfz-install plaits-install objectsdir=~/else-build
- Move the new Else folder (which is now available within your /home/pi/else-build) to the place where the former Else external was. If needed rename the former one for keeping a copy of it in case of (it's always what I'm doing).
- You are done!

4- Two remarks concerning the issues I was facing to:
- As Pd 0.51.4 was first installed from RPi repository, installing my compiled Pd 0.53.0 above it was making an issue with the its Dev-Headers.
For avoiding that, first clean Pd 0.51.4 installation by removing the remaining:
    puredata-dev/stable,now 0.51.4-1
- As the compilation of Else needs to have access to the ~/pd-0.54-0/src folder, make sure you have also available on your Raspberry Pi the source code of the latest Pd version you have used for compiling it.
And don't skip the 1st command given by Alexandre.

5- The ELSE compiled folder / nb of file 1083 / size ~ 55 MB for RPi armv7 (32-bit) is available on demand.
But it's missing both 'Live-Electronics-Tutorial' and 'pdlua' folders.

Best,
Joseph Gastelais