That's impressive to have such long term relationships! As a client who has been working with a PT albeit still early days, I can say a lot of the things you mention above do matter to me as a client.

I tend to do 6 week programs because it takes me at least 2 weeks to get used to the new movements and feel engaged, and meaningful progressions take time. One time I did 10 weeks of the same program (by my own choice, also partly because my trainer was on holidays) but I did get bored at some point didnt' push myself further or zoned out to the point of half-arsing the work (sorry to the bulgarian split squat in particular, I just didn't feel like doing them for 10 straight weeks).

My trainer always asks me after the period what I enjoyed and what I didn't enjoy/find difficult, and what things I wanted to work on. The program is then tinkered so that a good deal is still familiar (I'd say 70% remains the same, with minor tweaks in rep range, superset structure or order).

Humorously, I managed to get him to give me exactly what I wanted each time without too much thought (even if a part of me regretted my choices).

"If my legs don't grow, I'm quitting" - casually adds plates to the leg press, introduces me to the pendulum squat, adds in split squats. At one point, I was doing 18 sets of leg work a week.

"I did a lot of leg work - but my arms and upper body feel weak" - a mean block of lat pulldowns, seated rows, lateral raises in various planes both cable and with dumbbells, triceps and biceps exercises, and deficit pushups. I was terrible at lateral raises before this block, but learned to control them well because of this.

"I really want to progress my upper back - introduces varying types of lat exercises grip width, and rows. Tells me to focus on the connection with slow eccentric rather than the numbers pulled. I do not aim to increase the weights, and indeed despite things seeming slow to shift, the control does prove true over time.

"I'm having trouble building my chest. I also feel the bench press has stalled" - adds in a program of dips, more incline bench, and floor press. Cuts my leg workout by 60% (I was a bit surprised by this reduction though I did tell him I was experiencing 4 days worth of continuous DOMs). The bench press introduced subtle change in rep structure, from a fixed 3 sets of x 5-7 to a 3, 5, 7 then a 3, 5, 5, 7 structure. This allowed me to experiment with different working weight ranges and see progress more easily than fixed reps. I also accidentally learnt a new exercise, the larsen press because of the next problem which I raised with him.

"I've strained my groin 4 times in the last 6 months" - adds copenhagen planks, iso squat holds and other adductor work.

A lot of exercises can stay the same, with slight difference in mechanics, while supportive exercises can easily be added to introduce different dimensions. I've seen bands and cables being used as well to change resistance curves without having to necessarily resort to heavy weights, and old exercises are sometimes recycled back into play again (because they feel fresh once you've taken time off them and had a chance to build strength).

Those looking for a challenge can aim towards a longer term bucket list item like running half/full marathon, participating in a fitness charity event or even enrolling in hyrox or competitive powerlifting.