You could mount the ESP partition and then remove the old entries and kernels manually.
sudo mount /dev/your-esp-partition /boot
Change directory to boot.
Delete the menu entry to be discarded from /EFI/loader entries and delete the old kernel from the com.solus-project folder. Don't remove any other file.
Below is a minimal setup (normal Solus will automatically keep two kernels - the one in use and the previous)
Be careful not to delete your working kernel entry or working kernel files
Hope that helps.
├── EFI
│ ├── BOOT
│ │ └── BOOTX64.EFI
│ ├── com.solus-project
│ │ ├── initrd-com.solus-project.current.4.18.5-90
│ │ └── kernel-com.solus-project.current.4.18.5-90
│ └── systemd
│ └── systemd-bootx64.efi
└── loader
├── entries
│ └── Solus-current-4.18.5-90.conf
└── loader.conf