Comparisons between amateur players and pro's is like apples and lemons. Totally irrelevant. It's a pro's job, they put their body to the limit everyday, sometimes you go over that limit, and that causes injuries.
As for amateurs, the string stiffness has nothing to do with it. It's lack of decent fundamentals + overuse that causes injuries. Decent tecnique + moderate play hours prevents injuries. An amateur shouldn't put in that many hours. Also polys from today are a different beast than first gens form polys back in the day. Also if the tecnique is not very polished, there is no need to play poly, cause players in that case cant profit from the benefits a polyester brings to the table.
That said, if you want soft poly, have a look at Grapplesnake alpha, Signum Pro Xperience, Msv Focus Hex 38+, Yonex Poly Tour Air, Yonex Poly Tour Pro, Solinco Hyper G Soft, Topspin Cyber Flash, Volkl Cyclone Tour (1.3mm only) Ytex Pro Tour (if you are not from Europe), There was also Toroline Absolute but looks like they cancelled that one. IsoSpeed Cream, Head Lynx, Gamma Moto Soft and look into Dyreex brand, they got several soft poly offerings at great value for money ratio.
If all that should play still not soft enough, put Ghostwire or Gosen OG Sheep Micro in the crosses or even a cheaper Multi , Velocity MLT if you are on a budget.
Pro's Pro is the budget brand under the budget brands, but it'll loose tension way to quickly. you'd be better off with a string of the list i just mentioned.
Most of PolyFibre and PolyStar strings are also soft, but they got the same 'problem' as PP's strings. Tension Maintenance well below average.