Just a couple of thoughts if you don't mind my friend
On an arranger keyboard with auto accompaniment (styles) our left hand is playing by far less notes(actually it plays just the chords) than it does when we are playing a sheet music that is written for a piano.
Of course you know that the keys are not weighted at all, so they are too soft.
There are 4 types of keys for electronic keyboards of all types (arranger keyboards,synthesizers,digital pianos etc) if I remember well.
That's why if we just touch a little more a wrong key along with the right key with the same finger,unfortunately the sound will be heard by both keys.We are doing this many times
The various sounds (voices) of a keyboard and the combination of them, also can help to play a melody(or a melody line) with just one finger at a time without having a luck of the polyphony that would be more noticeable on a piano.
Just because let's say a real violin or a flute for example they also play a single note a time while the same composition in a piano sheet would required as many notes as possible to have a more "pro" result.
I don't know if by transporting every sheet music to the keys of C or Αm will be also helpful to you to play mainly in the white keys that way.
If a chord is hard to play you can "replace" it with a non that professional one(e.g Cm7b5 with Cm) but as long as you are having arthritis and you play just for fun, I suppose musicians with a good ear will not judge you too hard for this
If the sounds or the keybed of a keyboard from the psr E series don't satisfy you and money allows, you might search for a used keyboard from the psr S series in a good condition(buttons that will work with no problem and especially the first row above the keybed)
You better try some keyboards on a store if you can to see which one is having a sound that you like and a keybed that suits you.
Not weighted keybeds and more cheaper keyboards might have a less or more annoying.... "click-click" sound when we press them.