This is a great question and nice to e-meet you @smashradio! I'm Francesca from Fiverr Workspace Support 😄
At the moment, invoice number sequencing is somewhat limited. You can only enter in a number with no letters or special characters and the sequence would follow in numerical order. Unfortunately, it would not meet the needs of your example here.
This is an important note of what some of our customers experience though and I will submit it to our team.
We are new to the community forum so still cleaning up a bit, but feel free to submit this or any feedback in our Workspace Forum under Feature Request. We will be inviting more Workspace Members to upvote and share here starting in October and get these conversations going!