We have a POS app called “Sales Terminal” that has a cleaner interface than Shopify POS. It works either from your browser or from the Shopify Mobile app. We charge $10 per month per Shopify location with a 15-day free trial.
You can change the product quantity by typing the amount or clicking the up/down arrow buttons directly in your cart which is always visible. We also support exchanges, refunds, editing of old orders, receipt printing, increasing or decreasing prices, and Shopify discount codes.
Technically you can create a custom app and embed it in Shopify POS. Though your use case isn’t really supported, since the app wouldn’t be able to modify the UI.
I would have to experiment with this, however I have a hunch that we could use page links in Shopify POS to host a custom cart page that would pop-up with a quantity selector as items are added.
Again would take someone to think a bit out of the box to get this in your POS.
They used to have it so you could increase it right in the cart but ‘updates’ have reduced the size of the screen where you can actually see the products in the cart and the way you search and add to cart is very tedious indeed, when you need to increase quantity. I have no good work around yet, except to add my voice to the queue and hope Shopify sees it and reverts back to the more functional POS they used to have.