Rhododendrons have very shallow roots, as do azaleas. They're winter hardy, so the green effect is nice,and after a few decades can be quite imposing

however, that does not sound as tall as you'd like.
I don't know if you have enough room, but at my pool, the original builders created a 3-tiered terrace and planted a beautiful bloodgood Japanese maple that is now quite tall. Perhaps if you could create a raised bed along your fence, you'd have less chance of root interference. I'm not especially optimistic about lilac overwintering in MA, the root ball would freeze without sufficient ground insulation. One shrub that can be pretty vigorous in your zone would be rose of Sharon, but I will say they're a mess to clean from the pool

they're also quite invasive, as are trumpet vines, but in the right location (full sun) they can offer great screening in the summer. Again, if you raise a bed along the fence, you can boost the height a good couple of feet. You could then maybe do a row of miss Canada lilacs (lovely pinkish purple color and hardier than average) and the tallest rodos you can find, with a bloodgood as the centerpiece and maybe some tall cedar.
The problem, which you may have gathered, with tall trees 7' from the pool is not only the roots, but the limbs will eventually be overhead as well someday. This exacerbates a leaf issue and poses a lot of unnecessary risk from limb damage in a storm. I'm pretty sure the folks who planted my 70' tall maple weren't thinking of these things at the time, but trust me, ya don't wanna go there

pines are great year round screening, but a total pita when the needles get into the pump, which they will at even 3 times the clearance you have to work with.
But the Japanese maple even at full maturity like mine has a very compact foot print and if well trained are beautiful to look at with or without leaves
