Last year around this time I wrote a few Tiny Tasks.
They distilled my advice into quick things you can do while in the middle of the busy season.
They were well received so I'm going to start those back up, starting with today (but with a better name).
Today, get out a piece of paper, a pen, and your phone.
Open your Shopify store's homepage and start browsing your store. Go through your collection, search, filter, find a product or two, etc.
Every time you have to zoom in, close a popup, scroll to the right, or have to re-tap a link that you missed, jot down the page you were on.
All of these are mobile bugs from your theme's design. Each one is adding friction to your customers and harming conversions.
Now grab a different phone from someone else and repeat the process. (Optionally: give them back their phone when you're done.)
Send that list to your theme developer and help them prioritize their fixes.
One SEO feature that works quite well with mobile is structured data.
Mostly because the data helps Google's algorithm which makes their job easier.
You can either audit, code, and test your store's structured data by hand or install JSON-LD for SEO and have the structured data you need for several types of Rich Results.
It's already helped a few thousand Shopify stores with their structured data and it could help you too.