You might want to try the following, assuming your long term goal might be race pace of TT 1.1 with SPL equivalent of 16.
Warm up with some relaxed tempo lengths where you try to control your SPL and see if you can take it lower than normal.
e.g. try to swim 4x25 with decreasing SPL 18,17,16,15 and repeat once. Its a great skill to actually hit your target on each length as you teach your brain what a 15SPL length or 18 SPL length feels like each stroke.
I find this gives me a very fast tune up as the demands of increasing my stroke length give my muscles a stretch whilst my mind gets an immediate focus on balance and streamline.
It also starts to teach your body what an SPL of 17 feels like per stroke so that when you are training at your workout TT you should get some feedback as to when you start to lose stroke length and can compensate or correct.
Next take a main set similarly to how you describe above 25,50,75,100, trying to hold strokes per length. If you start to gain strokes try to analyse the cause. (with me its dropping legs causing drag).
Finally I would do some sprint work to teach my body the coordination of long strokes at faster stroke rates.
start TT at 1.20 and hold 16-17 spl for 25m, focus on reaching forward rather than pulling hard.
Once you have achieved 3 lengths at your spl target, take TT down 0.01/2 and repeat. Try to get to TT 1.1 or below and hold your SPL, depending on how much time you have.
End your session with an enjoyable cruise of 200m or so without TT and time yourself for interest, see if this goes down over time.
This is a bit of a cover all set and needs some time to complete, you are teaching yourself how shorter SPL feels at the start, working on stamina and consistent efficiency in the middle, adjusting to faster stroke rate in the middle without comprimising SPL and finally, developing a relaxed race pace that will improve overtime.
Let me know if you try it and it helps.
Think I will do it myself tomorrow, just found a 50m pool an hour a way that I didnt know about.