I feel your pain OP. I visually need my wrenches to match and taper by size correctly for me to enjoy looking at them. That's also why my most used wrench sets are always purchased in a brand with a healthy used market in case anything ever needs replacement. Luckily, I don't care if all my wrench sets are the same brand. I buy whatever brand makes the best particular wrench set for my needs.
If I were in your shoes and I couldn't find a suitable filler wrench, I would probably do one of two things. Since the correct 9mm wrench was made longer than the others, I would either stick the whole set on Ebay and buy a replacement set in a different brand, or I would throw away or repurpose the 8mm wrench somewhere else in the shop and remove the first two wrench slots from the storage rack. Within a couple weeks I would forget all about what I did and think it was always just a 10mm-19mm set of ratcheting wrenches.
For some reason I enjoy all the differences in brands, handles, etc. when I look at my plier racks and/or screwdrivers. I also don't care about everything matching when it comes to sockets. I have off brands filling in unmade chrome sizes, I have multiple brands of impact sockets filling in missing set sizes, I have a few 12pt sockets mixed in with my 6pt sets, etc. As long as I have complete and useful size ranges with no skips I am fine mentally.

Any sizes above or below what I find useful get their socket storage pegs cut off the end of the socket rack so I don't have any unused pegs. That's good enough for me.