Prompted to come out of lurking for this thread as it’s very much what I do day to day at work - assistive technology.
First, heed the advice above and ignore Doro phones. They are a waste of space and really only serve in making things more, rather than less, confusing.
Generally, an Android smart phone (doesn’t have to anything expensive or cutting edge) can be set up, with maybe 5-10 euros worth of apps and a little time, to meet the needs of pretty much anybody.
This usually involves setting up a custom interface using Tasker which will reduce the number of steps to complete something like making a phone call. For example, a call can be made by just tapping a picture of a person - no need to go into contacts, find the person’s name, tap the call icon and then confirm that you actually do want to call them!
The other good thing using such an interface is that they can be set up to work with taps. long taps or swipes - useful for people who maybe find a quick tap difficult to do.
Tasker can also do context dependent triggering of tasks which can be helpful in the case of someone who forgets to charge their phone for example. You can make it so that a phone will give an audible reminder when its battery drops below , say, 20% and will continue with the reminders periodically until the phone is plugged in and charging.
I could go on about this stuff forever! If you’d like some more detail here just let me know, or send me a DM if you’d like to discuss the specific case of your grandparents in more detail.