Good on you for being so determined to learn.
I have arthritis in both hands, it comes and goes and as yet is not too serious. On the days when it's painful, particularly if it's a gig day, then I just play in whatever way is least painful. Thumb if I have to, and then swap between fingers and pick.
If you do end up playing with a pick, don't consider it a fail. Musicians who say that playing with a pick is somehow wrong or shows less skill, should be ashamed of themselves. Its a totally relevant technique and for some music is almost essential and much more efficient than fingers or thumb. Ideally a bass player should be able to employ all of the techniques.