I like a crisp string in my racquets and multifibers can be nice when they're fresh, but not too much fun if they get soft and half used up before they snap. I stay away from poly to protect my older joints, but I know that they can be quite an addictive string option for harder hitters out there. Two options come to mind if you want to mildly soften things up for a bit.
The easy one is to drop tension in your poly mains a little bit and hybrid those with a multi cross. Some poly sluggers like a full bed of that string type, but lots of players definitely prefer the added feel and small degree of softness that comes with a softer cross. You could even consider multi mains with a poly cross, but finding good tensions with that setup could take a little trial and error.
The other easy option is to switch into a synthetic gut. Some consider this string type to be too basic to deliver decent performance, but I've been loving it for years and I string my own racquets. Aside from natural gut, I'm not terribly leery of the price tags on different strings, but this stuff gives me exactly what I like in terms of feel and performance along with enough softness to keep my arm in good shape.
I picked up some used Yonex RDS 002 Tours here some years back (98" head) and one frame came strung with a bed of Yonex 850 multi (16 gauge). The tension was somewhat firm and those strings exhibited very good durability for a multi, but after a few uses they softened up and that racquet became a trampoline for me. I had the same experience with the Yonex RD Ti-80 (also a 98") in that it played "big" and required a snug set of string to give me enough control. When I used a 16 ga. syn. gut in those racquets, I got enough of the firmness and control that I wanted along with a moderate degree of softness that need.
I get an extra degree of crispness, feel, and inherent softness with 17 ga. syn. gut, but I also understand that this thinner option will get chewed up more quickly by harder hitters or in frames with very open patterns. Not a problem for me, but I can't recommend it all the time. I personally enjoy Tecnifibre's syn. gut and I've also found Forten Sweet 16 to play especially soft among syn. guts I've tried out (their Sweet 17 was too fragile for me).