Nothing wrong with sticking to your existing set for a while if they are giving you good results. I don’t think you will find significant differences in performance between the G3 and G4 T350. They are significantly different aesthetically with the solid back and brushed steel finish which makes them very appealing, especially for people who play a mixed set. Everyone has different reasons and financial situations that impact their timing for iron upgrades. When you get ready, be sure and find a good fitter who will help you see the differences between old and new. Enjoy the process.

No you're not going nuts. There is nothing wrong with playing the clubs you get the best result from. Actually it's a pretty smart move.
To me, golf is a game that challenges you to improve your score more than the line of club you're playing. Playing the T250 or T150 you have to be able to hit the sweet spot consistently to appreciate them. I know more than a few single digits that play the T300 or T350.
Also, it's human nature that when you go to the golf shop and mess around with different clubs, you left your gamer swing in the trunk of the car.
I think you're at the point where you should consider hooking up with a Titleist fitting. They will let you warm up and establish your current swing, and then work with you.

I went from Gen 2 T300 to gen 3 T200 and now the gen 4 T350. Either T350 was a huge improvement over the T300. Especially on acoustics. This time I got better launch with the gen4 over the gen 3 T350.
There is no shame in staying with the T350 until or unless you are consistent on hitting the sweet spot. Moving onto other T series irons will improve shot shaping ability and adjusting flight. But forgiveness decreases as well.

The 350s are excellent irons. I have a GHIN of 14 and they have been good to me for the last couple of years. I have a friend with a 4 GHIN, and he plays the same clubs, although his are right handed and mine are left left-handed. My swing has slowed down and in need of more loft so I am switching to the 250* Launch Spec. When you think your irons are holding you back, you can try out some of the other offerings, but no need to change something that is working. Cheers.