Custom Omni-channel POS System Development

I have a need for a unique POS system that I would like to be developed. I need a POS system that will support multi-locations and omni-channels. I have quick serve restaurants (item modifiers, kitchen display screens, tipping, online ordering with POS and KDS integration, etc.), retail stores (inventory management, COGS tracking, item variations, print/email/text receipts, etc.) and boat rentals (online booking with deposits, up-sells, digital contracts, time tracking, etc.). I would like a robust back-end that will allow me to easily add items, manage POS devices, view reports, etc. I would love an open API to be able to integrate my data with other tools (Retool, label printing, etc.). I will need integrated payment processing via API to a couple of payment processors such as Ayden, strip, etc. We have a few special use cases like "house accounts" where customers can charge to a running balance (requiring a pin) to be charged automatically at the end of the month with an itemized statement.

We have been using Square for about 5 years and like most things about it. I would love to emulate their features closely with the ability to customize.

I am fairly technically savvy and would like the ability to make customizations/upgrades/updates on my own after the completion of the project.

I am sure there will be additional features as the process moves forward, but I would love to get an idea of what something like this might cost to develop and implement and a loose timeline.