I've had pretty good experiences with handing them Linux machines (usually openSUSE + Xfce). There's an initial adjustment period as they figure out "okay the orange fox replaces the blue e", but quite a few people I know who made that switch are now at least reasonably productive. Chromebooks are another option here that I've seen be pretty successful, and they have the advantage of being absurdly cheap.

The absolute biggest bang for the buck: ad-blockers. Fewer sketchy links to click on, and absolutely nothing to relearn.

Seniors minds are less flexible to change so a consistently exact OS with exact functionality is best even it looks out of date to us. I had to hire someone to exactly recreate my mom's desktop and and find and install her 15 yo apps. This kept her going with searches and email for another five years when otherwise she would just have given up. Also in-home tech people who are very, very patient. I'd happily pay $100/hour not to do that.