Following the Devuan installation guide,
https://www.devuan.org/os/documentation … all-devuan,
additional packages/software can be installed after step
"18) The installer will now install the packages you selected. This will take some time.",
before step
"19) Next you will be asked to choose your preferred init system. The default in Devuan is sysvinit".

The newly installed system is mounted on /target during the installation process and is fully functional after step 18 above.
You can chroot into the new system at this time and use apt-get to install any available package.
You can make almost any changes at this time you would be able to make after the system installation has completed and you've booted into the newly installed system.

Here is the procedure.
1. Start the installation and following the instructions in the guide.
2. Wait until step "18) The installer will now install the packages you selected. This will take some time." has completed.
3. The prompt for "19) Next you will be asked to choose your preferred init system. The default in Devuan is sysvinit" will be displayed.
Do not proceed with selecting the init system you want to install at this time.
4. Press Alt+F2 on the keyboard to get to tty2 then press the Enter key.
You will be presented with the root user prompt.
5. Chroot into the newly installed system by entering the following command at the shell prompt.
chroot /target /bin/bash --login
After which you will be logged into the newly installed system just as if you booted it up and logged in.
6. Install the desired package such as the firmware-amd-graphics package by entering the following command.
apt-get install firmware-amd-graphics
7. Install any other packages needed.
8 When done, exit the chroot by entering the following command.
exit
9. Press Alt+F1 on the keyboard to get back to the installation.
Step "19) Next you will be asked to choose your preferred init system. The default in Devuan is sysvinit"
10. Continue the installation from this point as described in the installation guide.

That way it would start immediately into the gui, instead of dropping to shell and leaving you with a problem.

I did this the hard way, once I knew what was required, before the end of install reboot, and it worked out a lot better than the first install attempt.

My graphics adapter is a Radeon HD 7700/7800 Series and needed the firmware-amd-graphics package.