Most visitors see your website on a phone FIRST, so let's look at it the way they do! Grab your phone, open your site, and work through each area below. You'll finish with a short list of fixes.
Try it without pinching or zooming.
Open the menu and move between a few pages.
Service times, address, and contact, within two taps.
Turn off wifi and use your phone's data (no cheating!).
Try everything with your thumb.
Pick the three that will help visitors most.
Everything you flagged, plus your top three. Copy it or save it as a PDF for whoever updates your website!