Hi John and Haz,
Shopify is so great for small businesses that want something working out of the box. It takes away the problem of hosting, security, maintenance etc but the flip side is there is loss of flexibility. So John’s contractor might well be justified saying there are things you can’t change on Shopify.
As Jerome says, Magneto is advanced. If your business got used to Magneto flexibility, you might find Woocommerce a good compromise now. If you go for the self hosted flavour, you can add numerous plugins and/or custom code. You didn’t mention the specific technical issue you need flexibility with, but I specialise in Woocommerce so if you’d like a quick call to get some advice, feel free to DM me through my BBF page.

@Haz, we haven't moved yet but mainly because the contractor we engaged let us down on the agreed deadlines. It is due to happen in Feb 2021 so for now I'm afraid I can't give information on how useful it is. First impressions are that we're going to have to make a lot of compromises on the flexibility we're used to having. Maybe I've got the wrong developer but a lot of queries come back "You can't change that on Shopify"...!

On an unrelated matter, can anyone recommend a good Shopify developer? Asking for a friend...

Hi John, good to see this post. Did you go with Shopify in the end? I'm looking for a similar, singular type solution for product sales and stock management, both online and instore. So far Square seems to be good candidate for this: squareup.com. Haz

Yes I must say I've used Shopify on various projects and it's excellent. Magento is very advanced and needs quite a bit of resource - possibly more suitable for larger ecommerce projects. I would say Shopify is a good option and I'm also hearing good things about Wix