Not sure if this helps, but I did something similar with our part numbers where I needed to start the number with an alpha prefix based on what the user selected for the part type. (We were generating these in a separate Access DB at the time). I created a few different sequences, and then since we are on-premises I was able to cheat by seeding the Ice.SysSequence table with the highest value already used for each of the part types.
This won’t help fill your gaps, but maybe it helps going forward?