Turning 69 this year, so thinking I may need to replace my motorcycle with some kind of manual shifting sport type car. Thinking about a Mazda Miata, but any other suggestions would be helpful. Have a RCF for a daily driver, but want a car with a manual transmission. Don't want some high miles, expensive to maintain car, but wiling to spend some bucks on something fun to run around on the back roads out here in Texas.
Any thoughts on what car could replace my motorcycle, and still have some type of driver feedback that makes driving fun?

Might want to look at the Fiat 124 Spyder. It has most of the reliable Mazda mechanicals, except for an Italian-sourced engine, and, IMO, is better-looking, front and rear, than the Miata. They were co-designed, except for the engines and front/rear styling, as pretty much the same vehicle, on the same platform. The main problem with the Fiat, though, is its sparse dealer network in the U.S.

Miata
brz/frs
golf gti
s2000

Your initial idea was spot on. A Miata is exactly what you're looking for.

The manual is the hard part, even used. If I were you, and I am assuming cost is an object (it is for 95% out there), I would say base Corvette, new. If cost were no object, the new Porsche 992.

Miata
Is
Always
The
Answer

Unless you're well over 6' like I am, and then it's hopeless. I like the Corvette/911 suggestions as well, or perhaps an M2 or M240i.

BRZ/86 would be my choice for new. Miata if you want the open air feel. The Miata RF is a unique experience as well. You already have the RCF for high power shenanigans.

The bike is really a Sunday driver....so does the replacement have to be new? If not, again, Corvette, used. If a RCF is a daily driver, would a Miata be satisfying to drive? Maybe....but maybe not? I know in driving my wife's uncle's Z3, I didn't really find it enjoyable. Whereas a Vette that I test drove was. As was the 991. Just me, a Civic sized car is about as small as I'd want to go...

https://www.toyota.com/86/