I tried a few apps but always ended up having to modify the listing on Amazon because of the different format and different customer base.
Here's my workflow with Amazon / Shopify:
1) Create products in Shopify in the admin
2) Create products in Amazon using their spreadsheet upload. This allows us to tailor the content specifically for Amazon customer vs branded store. With the template upload, you can bulk upload products in spreadsheet, which is way faster than creating in your seller central portal.
3) Use Amazon FBA to fulfill Shopify orders, and set it on autopilot so orders come in and are automatically processed and fulfilled (details on that step here: https://speedboostr.com/automate-order-fulfillment-on-shopify).
Through lots of testing, I've found that different prices and content convert differently on Amazon vs Shopify. On Shopify, it's your home base so you can add extra content / graphics, and links to other products. On Amazon, you're competing with everyone, and there's a certain format that works better there (like highlighting your bullet points, using more salesy copy, etc).
Cool in that case ya I think Codisto is still your best solution. The price makes sense to me, I've done some work with the Amazon API, it's a pain to develop with and it costs extra money, so the cost to build and maintain an app around it is going to cost more.
It's not something we'd build because there's already a solution, but another idea is you could try is contacting Codisto support and share your frustrations. Maybe they can create a special plan for you with limited functionality to better serve your needs with more value. I've worked with their support before (I'm also a user of their app with a store), they're pretty cool and I remember them helping solve a challenging problem we were having a couple years ago.
Good luck on the search, I hope you find your ideal solution.