The RP 14 and RP15 are brand new Rheen mid-range models. They were just introduced earlier this year partially to comply with the new 14 SEER minimum in the Southern states. Almost impossible to answer questions about reliability at this point.

What model Rheem heat pump do you have now? Rheem makes some great long lasting heat pumps but unfortunately also makes some flimsy builder grade "value" heat pumps. If you have one of the better ones, it might be worth finding a way to keep it going for a while by making duct improvements, for example.

What sq ft are you cooling? I think you are an excellent candidate for a load calculation, especially since your current system is not cutting it. Maybe the issue is ducting rather than tonnage. I would guess your air handler and ducts for the second floor are in the attic. Having multiple returns on the second floor is important.

Does you power company offer rebates on home energy audits? Last year, our community had a deal for $50 where they came out and looked at everything and made recommendations. Of course, some of it is self-serving upsell like new windows, but if you filter that out, they give you some good information.

Thanks for the response. Ok, so no long-term reliability answers. I thought it might be another proven system under a different name that folks were already familiar with. It looks like it's manufactured in a nice, newer facility in Mexico.
We currently have a Ruud UPKA-025JAZ manufactured in March 1999. The fact that it's still working fine after 16 years makes me think it may be one of the good ones.
We're cooling about 1200 sqft, on the second floor and it's open to the downstairs via a central hallway with a balcony that looks down on the foyer.
We have lots of vents and 3 returns spread out on the second floor (20x20/12x12/12x12).
The ducting is definitely older but holding up well. I've walked through the attic checking for leaks and there's nothing obvious. The blown-in insulation is really good as well.
We can/should get an energy audit from our local provider and that's a great suggestion.
Really just debating upgrading the system and going with a higher SEER (10 to 15). Currently, the fan runs non-stop during the day with the thermostat set on 78 and the system keeping it around 80. Thankfully, it's bearable at night and we have the luxury of taking our time to make a decision.