Very helpful, thanks!

I think any "set" on the market will be fine on any more-or-less standard dulcimer in CGG, DAA, DAD and DAC tuning. There is no reason to try to out smart McSpadden, or the other industry leaders

That is assuming you start a bit low and carefully tune up to the note you want. A beginner can accidentally tune a few turns too high too quickly and break a string, and I think part of the selections used in making sets make an allowance for that.

I think you can go up to 14 on the melody strings if you stick to DAA, but 12 is safer for DAD, and 10 requires less finger pressure. DAD players typically finger about 3 notes on the 14 middle string, anyway, so 14 A is playable.

Finger pressure is also dependent on string height which is also a personal choice. Have you mastered checking and adjusting that yet? Builders often leave strings a bit high to allow easy adjustment with a small file if they don't use a zero fret.

Assuming I wanted to stay Ionian but three strings only, would 22-10-10 be a concern?  I haven't looked elsewhere, but the only Ionian string set sold by McSpadden for a standard mountain dulcimer is 22-12-12 (with an extra 12 if I stay with three strings).  Similarly, if I wanted to go Mixolydian would there be a drawback to 22-10-10?  The McSpadden Mixolydian string sets have a 23-14-10 (with an extra 10).  It seems that they consistently want different gauges on the 2nd (middle) and 3rd (melody) strings for Mixolydian, as opposed to, matched gauges for Ionian.  Is the higher gauge on the middle string intended to be more compatible with the increased tension in the 10 melody string when it is tightened to reach D?  Is there a consensus that a 10 middle -10 melody is not good for for Ionian?  I don't think so, but I thought I would ask.  The 10 is easier to fret in Ionian, so I hope the answer is "No"!  Thanks!