First, let me say I too am impressed with your working consistently with a single piece for so long. I have a 19 from that era-youngest daughter has it now, and is a Gunsite graduate with it. I bought her another Glock-a Gen4-awhile back so old faithful can rest.
@GJM set up a 26MOS similar to your thinking, except I believe he used an EPS. The description is at the end of the G26 thread.
I have a 4 or 5 year old 5th gen 26 with irons, and while it doesn’t have a high round count~2,000 rounds or so, it’s carried regularly, with no problems. It shoots better than I can with irons now.
I don’t have a ton of time on the 509T, but several folks at work have run them and had no issues. One instructor really liked the green version,FWIW.
In short, I think you propose a solid combination. My only suggestion would be to look at an ACRO footprint, but I am partial to that mfg. There are other sight choices with that footprint: C&H “DUTY”(our sample working well), a newcomer from Lead& Steel, the PB-3(looks good, but we literally just got one to test)and I think the Steiner MPS will work on that cut(an ACRO plate fit).
Best of luck