as a long-time Shopify Partner, it's tough for me to comment on the review process because all my submissions are expedited (24-48 hours).
here are some general pointers instead.
Red Oceans
the Shopify App Store is a race to the bottom. do not -- do not -- build a clone with 40 competitors. stuff like: receipt printer, coupon bar, out of stock tickers, etc.
you will be competing with free. you will be competing with 100s of 5-star reviews, earned over several years. and yes, even simple innovations like these have 69 extra "all-in-one" features to justify their existence.
Pricing
every indie hacker reads the same advice: "charge more!" then they build a weekend MVP, slap $49 /month on the tin, and wonder why nobody cares.
understand that in Shopify -- because all listing pages are the same -- you only have ~4 opportunities to differentiate:
i reckon design and reviews are the most important. yet when you launch an MVP, the design probably sucks. and you have 0 reviews.
so here's the trick. launch with a cheap price and extended free trial, then make free customizations for early users. politely ask the most satisfied early adopters to write a review, then increase the price and shorten the free trial.
every 10-20 new reviews you receive... rinse/repeat.
note: price changes on Shopify only apply to new installs. you cannot force ugprade an existing install to a new price a la Stripe.
Audience
it's too simplistic to say that ecommerce apps serve "ecommerce people." like any product your target customers may be marketers or developers, customer service reps or accountants.
thus before building an app, think about who the user will be, and what their skillset will be too.
for example. when we launched Picture It, a live preview AR/VR app for art stores, we failed to think about the customer persona beyond "art stores." meaning, i forgot that artists typically have low/no tech skills and need hand-holding to onboard.
so while our app is literally "Live Preview AR," we get support tickets like "does your app provide live preview VR?" and if a customer needs to customize their HTML? forget about it. i personally implement HTML/CSS tweaks on their behalf, free. #facepalm
Summary
If you build a quality app, with low to medium competition, and respond aggressively to customer requests to garner 5-star reviews, the Shopify App Store can be a money printing machine. but if you miss any of these points, you lose.