Ashridge, when you demo racquets, please keep in mid that you swing a racquet, but hit a ball with the strings. The impact that your arm and shoulder feel comes from the impact of ball against the strings and partly from vibrations of the string bed after the impact. The impact of strings on how your arm and elbow feel is huge, and yet demo racquets are likely to come with lower end strings, and oftentimes with dead strings. As an owner of a stringing machine who used available opportunities to learn about strings and stringing and did a number of tests, I can say that I reached the point when the only meaningful racquet test for me requires that I restring a demo racquet with a good quality string (my local tennis shop allows this, but this of course comes at a cost as they would not pay for premium strings, like natural gut).
Dead poly (as you know, it takes about 8-10 hours of play time, or several weeks of storage time, for poly to elongate, lose flexibility, and become dead) is the worst for your arm and elbow if you are sensitive. Natural gut is by far the best. There are many multifilament strings that are in between. Proportional stringing reduces vibrations quite a bit and further reduces impact on elbow.
I had a tennis elbow a couple of times and I have a high level of sensitivity. My good friend ran into the same issue last season, with poly strings recommended to him by his tennis club stringer. Both he and I now use natural gut strung per Sergetti method of proportional stringing (I installed them for free, he only pays for the cost of strings). Since last summer, when we moved to this type of stringing, all elbow problems are gone. However, when I try a demo racquet with dead poly, I feel something in my elbow right away.
A hybrid with natural gut on the mains and poly on the crosses is also a viable option, as it could slightly increase spin and control, but will not take too much toll on your elbow. The problem is that you never get strings like that on demo racquets.
Where I am going to with it, you surely should find a frame which feels comfortable weight and balance-wise, but the ultimate solution for your elbow and shoulder lies in strings. Parameters of the racquet which help with absorbing shock is weight (the heavier the better) and flexibility of the racquet (but this is a trade-off with control, and the range is relatively small, anyways). Obviously, if you are determined to use stiff strings (like poly), you may have to go with the most flexible racquet, but if you are open to a variety of strings, you might as well get all arm friendliness that you need from the strings.
I've been using RF97A since last summer, and when I read "Loved the way the RF97A felt when hitting, but it just ended up being too stiff and too heavy for me over the course of a match" in your post, I can tell with good confidence that "being too stiff" most likely comes from the strings. There is nothing too stiff about this racquet with more elastic strings, in my opinion. The "too heavy" part is of course your personal observation and it is valid for you. Wilson makes lighter racquets in ProStaff series, and Blade and Ultra are also worth trying. For me personally, Blade is the second best after RF97A, but at your tennis level you obviously should only rely on your own feelings.
You might also find, when renting the same racquet for the second or third time to dial-in your choice, that each time it feels differently, sometimes like crap, sometimes great. This is not you and your mood, this comes from what strings are installed on it and on their condition.
P.S. You wrote in one of the follow-up posts that you string your own racquet(s). You did not discuss what strings you use, so I might suggest that you try stringing your existing racquet with natural gut or natural gut based hybrid to start with. You might find that this would resolve your issues and you might not even need a new racquet. Also, based on my personal experience, I would recommend that you use a Sergetti tension sheet - but you need a constant pull machine to proportionally string correctly.