flutterby, welcome to MM!
I can understand your dilemma. Having had breakthrough bleeding and spotting on your current regime, I can see why you would want to change it. I do think it would be a pity to stop taking HRT altogether when you retire, it would be nice to feel at your best when entering this new phase of your life.
It must have been worrying to have the build up problem but good that you have had it checked out and all is OK. I agree with DC and Taz2 and would suggest a separate transdermal bio identical oestrogen with bio identical Utrogestan progesterone for either several days per month (cyclical) or every day like Taz for a continuous combined regime. As Taz said, this is a perfectly safe form of HRT that carries very few risks and there is no reason why you can't take it for several years and reap the health benefits that it offers. Having been on the 50mcg patches, you could probably get very good results with just one pump of the Oestrogel every day. The gel is flexible and you can adjust your dose to suit your needs. The Utrogestan is more effective when taken vaginally and you may find that it stops your spotting and breakthrough bleeding - I only had breakthrough bleeding when I tried to go for a long cycle.
Being a scientist, you probably already know this but re the lining measurement problem, it is worth noting that women who take cyclical HRT will always get a greater build up than those using a Mirena coil or taking continuous combined HRT. When I had my uterine ultrasound scan the other week, the gynaecologist was saying that the thickness level depends on where you are in your cycle and how much oestrogen you are taking - obviously, the more oestrogen you take, the greater the thickness mid cycle (it is thickest from day 13 to 23 of the cycle) and the heavier the period will be. Where were you in your cycle when you had your reading of 6.3?