I know plenty of native/fluent signers that finger spell slowly. Doesn't stop them from using the rest of the language though.

Don't let that stop you.
Funny you mention the hand issues as I have arthritis. I know you said you don't, but it's possible the tendons or muscles in your hands get stiff. I find running my hands under hot water, even just for 5 minutes, to be really helpful. I take anti-inflammatories and muscle relaxants for TMJ/arthritis, which help with my body in general, including my hands.
I can't form the R shape so my R's look like U's. Sometimes I bend my middle finger down instead of trying to cross it over the index finger.
I also have a hard time with 1 handshape so I sign closer to a D. And I can't fully sign a W either but I can get my pinkies and thumbs close enough so that it is W-ish enough to be read as a W. I can't bring up my finger for X on its own so I often sign the handshape you use for "writing" and "changing".
Lastly, with my wife, I use the modified BSL alphabet for the Deafblind because it requires less movement than the ASL modified alphabet.
Video of BSL alphabet for the Deafblind:
BSL alphabet for the Deafblind
Description of DB-modified BSL alphabet