Firstly, extlinux itself only knows about the one partition that the bootloder resides in, which in your case is /dev/sda1. Therefore all pathnames that you use in the menu declaration are pathnames into that filesystem, which basically is the /boot directory tree without the /boot prefix.

* Does that filesystem have a pathname /vmlinuz that identifies the linux kernel to load?
* Does that filesystem have a pathname /initrd.img that identifies the initrd to use?
* Is that filesystem really a root filesystem?

From you ascii graphics it looks like the /dev/sda1 filesystem is normally mounted on /boot of your actual root filesystem, which you spin up from an encrypted logical volume within the /dev/sda5 partition, and it's rather there one would expect to find those pathnames /vmlinuz and /initrd.img (as symbolic links into /boot).

They are thus not available for use by the extlinux bootloader. Rather you need to provide the pathnames to the kernel and initrd as they are within the /dev/sda1 filesystem; it would be something like /vmlinuz-6.12.95-amd64 and /initrd.img-6.12.95-amd64, depending on which versions.

Note that the toplevel of that filesystem (on /dev/sda1) is "normally" mounted on /boot, and the /boot prefix is part of "the normal root" filesystem rather than of the filesystem on /dev/sda1.

The third problem is that your declaration tells the booting kernel to try to mount /dev/sda1 as root filesystem, which of course fails. Rather, you need the initrd set up the decrypting device /dev/mapper/sdb5_crypt for the logical volume system to use, and then your actual root filesystem would be found using the LVM name /dev/userr-vg-root (or similar; I don't use LVM).

In parenthesis, I think it's confusing to use name sdb5_crypt for an encryption of /dev/sda5, especially as you apparently also have an /dev/sdb.

Anyhow, other than that confusion about pathnames it should all work fine.

Btw. please use bbcode markup around "code" portions to make them clearer.

Welcome to Dev1.

Have you tried installing Devuan with the default/standard [GRUB] package?
If so, was there something that you found lacking with it?

I have been running Linux in my boxes for ~12 years (Devuan since 2017) and have never had any issues with [GRUB].
To the point of being practically transparent to me, except for custom boot screen configurations I needed.

Best,

A.

Edit:
You will get valuable insight from this post by [fsmithred].
Point to remember: the forum's [Search] function (between [User list] and [Profile]) is your best friend.

My partition table looks like this:

NAME                       MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
sda                          8:0    0 931.5G  0 disk
├─sda1                       8:1    0   976M  0 part  /boot
├─sda2                       8:2    0     1K  0 part
└─sda5                       8:5    0 930.6G  0 part
  └─sdb5_crypt             254:0    0 930.5G  0 crypt
    ├─userr-vg-root   254:1    0 914.6G  0 lvm   /
    └─userr--vg-swap_1 254:2    0    16G  0 lvm   [SWAP]
sdb                          8:16   1  28.9G  0 disk

As for the /boot/extlinux/extlinux.conf file it looks like that:

ui vesamenu.c32
menu background splash.png
timeout 50

label devuan
    menu label devuan
    kernel /vmlinuz
    append initrd=/initrd.img ro root=/dev/sda1

I've tried to change the root to: /dev/sda5, /dev/sdb5_crypt and userr-vg-root but it didn't help.

Any ideas of what when wrong?
I also want to say that this is the first time I'm trying to set a new bootloader.