Unfortunately your deployment will interfere with something to slot above your 5 iron. Besides the T250 there is also the T150, a U series driving iron, a hybrid, or a 7W.
No matter works for someone else, there is no reason that will be your best option. The safest/optimal solution is to get comfortable with your 5 iron and look at the distances between that and driver. A fitter can recommend a 4 iron based on the results with your 5 iron. You can also pick one of the other options as you can get fit for them and decide which meets your shot window and distance need.

I had exactly the same issue. I played AP1 irons for a very long time, basically the ultimate game improvement shovels, and recently had a fitting at Titleist in Munich.

I ended up going with the T150 from 7 iron to GW, and T250 in the 5 and 6 iron. My main issue was always a very low ball flight. With the T150 I’m finally getting a much higher launch, even though they are a bit shorter than my old AP1s. But honestly that is not really a problem, you just take one more club and gain much more control and consistency.

I currently play off 2.6, but when I was fitted for my AP1s my handicap was significantly higher, so my game has changed a lot since then. For me the combo setup was definitely the right choice.

Thanks for taking the time to reply. I haven’t typically struggled with height/trajectory but I’m getting older so time to put ego aside. I’ll try a few options here and there but nice to ‘now the T250 are working for you in the longer irons

I don't think the jump will be too large. A lot of players blend the T100 and T250 long irons for exactly that reason. The T250 4 iron is designed to launch a little higher and provide more ball speed so it hits that launch window . It can be a nice option for longer approach shots and off-the-tee use.

The biggest thing I'd watch is the loft gap. Depending on the specs of your T100 set, the T250 4 iron may only be a few degrees stronger, so you could end up with a distance gap that's smaller or larger than expected. Without a fitting, I'd compare the lofts and make sure the gapping makes sense.

As someone who works in club fitting, the T250 4 iron paired with T100 5-PW is one of the more common mixed sets I see. The larger concern isn't whether the clubs blend well, they do. It's making sure the shaft and loft setup produce the distance gap you're looking for once you're able to test them.