It is a knotty job indeed. I went with malviez 's scenario and ended up with 1 GB ESP partition - just in case of an attempt to triple-boot in the future. SUCCESS ! 😃
Later, I realized that reinstalling the whole system is the most efficient way. My Windows partitions was a mess and it was very difficult to expand my Linux partition. Doing backups, reinstalling Windows and Solus, then restoring is much, much, easier for a newbie like me.
For those with the same problem that ended up in this thread, reassess your priorities if getting the cool badge mentioned by davidjharder is worthy of your time. 😁