Conflicting options - should I get a third?

Not according to my readings. Most oncologists will say Gleason 6 will not metastasize. Yes, cancer cells, like all cells, change over time. It is just as likely (probably more likely) that your cancer is from a cancer cell(s) growing over the 10 years.
I agree your urologist is a moron, but not for that reason. It is for his interest in removing the prostate right away. Until you do a PSMA Pet scan you don’t know enough to decide on a treatment. If you remove now, and it turns out it spread past the prostrate, you’ll need to do radiation as well.
I’ve had Gleason 8 diagnosed since January. Confined to the prostrate (Pet scan). On hormones that will shut down the cancer allowing me to spend a lot of time researching what I should do and take a 3 month trip to Europe. PC survivors, my 1st and 2nd oncologists agree that is the case. Now I’ll do Proton Beam therapy after I return.
Please don’t knock yourself. Take the time to make the right decision.

Not according to my readings. Most oncologists will say Gleason 6 will not metastasize. Yes, cancer cells, like all cells, change over time. It is just as likely (probably more likely) that your cancer is from a cancer cell(s) growing over the 10 years. How could you conclude it is the same one w/o genetic analysis?

I agree your urologist is a moron, but not for that reason. It is for his interest in removing the prostate right away. Until you do a PSMA Pet scan you don’t know enough to decide on a treatment. If you remove now, and it turns out it spread past the prostrate, you’ll need to do radiation as well.
I’ve had Gleason 8 diagnosed since January. Confined to the prostrate (Pet scan). On hormones that will shut down the cancer allowing me to spend a lot of time researching what I should do and take a 3 month trip to Europe. PC survivors, my 1st and 2nd oncologists agree that is the case. Now I’ll do Proton Beam therapy after I return.
Please don’t knock yourself. Take the time to make the right decision.

I think that you are mixing my case with somebody else's , but that is OK .

My husband's 3+3 became 4+3 over time - same lesion in question was 3+3 10 years ago.

Anyhow, my husband had PSMA and all other tests necessary, all contained in a gland and both radiation oncologist and robotic surgeon in center of excellence agreed that for my husband RP is the best way going forward and they both mentioned "cure" since he has only one core of 4+3 out of 14 biposy cores taken.

Because he has cribriform cells and IDC in that one core, the best course is to have RP. That way if he ever has recurrence he will have an option of having radiation. You can have one area radiated only ONCE. We wish to have back-up plan if it is ever needed.

BTW, radiation can cause multiple side effects, some are immediate and some appear over time ( even years down the road). Basically - there is no procedure that does not have possible side effects. Some people have them, some don't.

For us the main goal is getting rid of the cancer, side effects will be there or not be there - that is out of our control and they will be dealt with IF they happen to be there.

My wishes for the best results for you and your husband! You are both determined to beat it, which I think is probably a great indicator that you will!

The one very good thing about the prostatectomy is it puts all of the important cards on t he table. Especially in these days of PSMA PET scans which can detect whether some cancer has already slipped out of the prostate. It's not foolproof.
But ideally your post prostatectomy pathology report will show negative margins on the outside of the capsule (non slipped out by that measure), a very precise measure of the cancer-they slice and dice the removed prostate and eyeball the cancerous material very closely (hence prostatectomy pathology reports often turn expected Gleason 4+3s into more favorable 3+4s). Also get a first cut guess on whether any has slipped out to the lymph nodes (they take a few nearby and examine them). The procedure is quick; recovery a bit painful - bowel movements painful for awhile cause the operation is near it. Also your erectile function may be more readily preserved if the prostate is removed sooner than later.
Thoughts of a layman who had a letter perfect prostatectomy pathology report but still wound up with biochemical recurrence a year later. But even given that I ask myself how much worse that lymph node spread would've been if I had let its factory in my prostate chug along longer.

Just a thought based on my family's experience- my grandad had his prostate removed at age 71, he didn't have cancer, but as his wise urologist said-if something's becoming a problem it's best to remove it before it becomes a bigger problem. He was one month and 14 days away from being 100 years old when he died (and his death had nothing to do with cancer, he died from kidney failure after being prescribed wrong hypertension medication). His brother also had his prostate removed-he died at age 96 from stroke. And my father had a really lousy urologist who first did a TURP for benign enlargement (on MRI, SCAN, biopsy, just based on his PSA level-4 at the time), after which he deemed my father cured?! and told him to check his PSA once a year!? Well, you know how it ended.. I wish he had his prostate removed back in 2016, he would still be here today...
Anyway-best of luck to your husband, hope he gets to be a 100 like my grandad!❤️