Makeup brushes are sold on bristle type, density and shape, and for anyone with a painful or weak grip almost none of that matters. The binding constraint is the handle, and the handle is the part nobody describes beyond its color.
The standard makeup brush handle is a thin tapered stick, usually round in cross-section and narrowest at the point where you hold it. That shape exists because it mimics an artist's brush, and it is close to the worst possible design for a compromised grip: it requires a pinch between thumb and forefinger, it has no surface for the hand to rest against, and the taper means it tends to rotate under pressure.
The alternative is well established in other product categories and it is the same principle adaptive cutlery uses: a flat, broad handle. A wide flat handle can be held against the fingers and stabilized with the palm rather than pinched, which both reduces the force needed and increases control. A handful of brush ranges do this, usually with flat bamboo handles, and they cost no more than conventional ones.
The other half of the answer is to use fewer brushes. A sponge held in the palm needs no grip at all — it is the most accessible tool in makeup, it requires no pinch, no fine finger control and no rotation, and it happens to be the best tool for mature skin independently of accessibility, because pressing deposits product on the surface while a brush swept sideways drives it into lines.
A practical note on how many tools you need: two. A sponge for base, and one small tool for the under-eye and for placing concealer on a spot. Twelve-piece brush sets are mostly eye brushes that will never be used, and more objects is more difficulty, not more capability.
