Might try a USPRT type set with some modifications.
At :10 rest you're holding 2:20 leaving on the 2:30 or so for 100s at 18 for 45 min, which is about :35 per 25. That's what I'm basing all my math below on, adjust if it's too easy
Try swimming the 25s or 50s . 25s hold :30 leave on the :40. Although if you have a tempo trainer I think a better send off would be :38-:39. That's just harder to manage on a pace clock. You can also use the tempo trainer to make sure you're swimming these all the same pace.
for the 50s hold 1:02 leave 1:10
If you did 75s hold 1:35 leave 1:48 if a tempo trainer or 1:45 or 1:50 on the clock
this would allow you to hold a higher velocity for when you're swimming which should in turn help you get to 19 then 20 100s in the same amount of time over the next month or so.
If you're finding that you feel kind like death on these, do 8-10 repeats, take an extra :20 between groups of how ever many repeats, then start again.
Hope that helps
Brian Stover USAT LII
Accelerate3 Coaching Insta