Depends what you want to display.

My Garmin Edge 800 (a refurbished model bought on-line, now being phased out by Garmin, so significantly less $$$) can certainly be tailored to help legibility:
(a) brightening the screen (while this impacts the battery life, I have not found battery to be a problem even on a long one-day ride);
(b) vary the number of items (speed, gradient, heart rate, time, cadence etc) displayed on a single page - font sizes adjust to fill the page. One can also specify exactly what is displayed on a page, and how many separate pages are displayed.

However, other aspects seem to have a fixed font size - such as:
(a) main menu
(b) direction cues when being used in 'navigation' mode - albeit one can 'zoom' the navigation to change the scale of the map (and again the map changes to fill the page so legibility increases).

I need glasses - my prescription sun glasses seem to make the navigation display harder to read - perhaps the 'anti-glare' properties are a factor.

My recommendation if you are concerned - go and try it out. If you are uncomfortable 'using' your LBS if 'on-line' purchase being contemplated, there are lots of cyclists using Garmins, most love to talk about their equipment. The web-site operated by DCRainmaker has very detailed assessments of bike-related equipment

Cell phone can certainly be used - but if you keep the display on you likely want an extra battery pack for longer rides.

There is also a separate forum for electronic stuff - some of the folks there are very knowledgeable, and can also fill in on all the alternatives