First, I would recommend going to a Titleist fitter, I have the T150 7-P and T250 5,6. Based on my fitting, my T150s are bent 1 degree weak and my T250s are bent 2 degrees weak. This essentially means I have T100 lofts throughout the bag, and my ball flight is unlike anything I've had before. It's really awesome to see from my perspective. Also, the new fitting heads allow the fitters to experiment with so many different options to get you dialed in.

Hope this helps!

Here’s my solution, T-250 4 and 5, T-150 6-9, and Vokeys there after. There’s really no gap … you can always grip up or down, or play draws or fades when distance needs adjusted. But the matched set is solid!

Anyone looking at a combo set without a fitter is like playing the lotto.
The different models each have unique characteristics, so the degree difference will most likely have an odd gap where you make the switch.
Go to a Titleist fitting and let them tell you.

Not sure how you have gapping problems when you don't have the clubs. But...
The degree difference isn't like for like. They're completely different construction and aren't comparable like for like based on loft.
Only paying for a proper fitting by a club builder will ensure your gapping and combo set are correct.

Adjust the T-250 6 iron to 28 degrees to create 4 degrees between all clubs. That change will add one degree of bounce to the 6 iron which shouldn't have a significand effect on performance.

Titleist sponsored fitters will determine your primary head based on your current clubs and the fitting. Based on achieving 5mph ball speed increase with next longest club, the fitter will then offer options to maintain that increase.
If the mixed set has a distance gap, it can be adjusted with lift adjustments.
Pretty sure it is Beau Hosler that has 2 5 irons (different series) with loft tweaking for gapping.
Why guess empirically while spending $1500 if you get the fitting for free with a purchase?