What usually works for me is:
- Boot the solus live session
- Remove the ESP / Boot flags temporarily from all other relevant partitions using GParted, KDE Partition Manager or any other suitable tool (you might have to set another flag as some oft these tools do not allow the flag to be empty).
- Then run the Solus installer and chose the drive you want it to install to. It should now also select your chosen drive as the location for the ESP partition.
- Complete the Solus installation.
- Add the proper ESP / Boot flags back tor your other partition and reboot.
Now you should be able to just pick which OS to boot from your UEFI settings.
If other ESP partitions are already present when the installer is launched, it will just pick the largest or first one it finds IIRC and not create a new one.