If my company came to me and suggested we do something like this, I would urge them not to do it.
I suppose it could work just fine, but then you have to ask yourself if recreating the wheel is both worth the effort, and maintainable. I don't know how many/big is a "very large" customer list, but:
You'd be recreating your CRM and your ERP via O365 webapps; if that's not daunting enough for you, then I suppose you can do it. You can use Power Automate to take care of those repetitive tasks, Power Apps to add/update/delete things, Dataverse/SharePoint to store things... You'd consider next data standardization between the two existing systems, then possibly merging your two customer databases (unless there's a good reason to keep them apart, I suppose). From there, outline how your tables will be setup.
You'd probably need to work way harder than you want to just to get similar functionality of an existing system (not saying that you cannot get the same functionality, just that you'd definitely need to put effort into it).
Then what happens if you leave? Are there multiple people in your company developing this? If there's nobody to maintain it, you could/will cripple the company (e.g. you're out sick, you leave/retire, etc).
Sort of got on a rant, but in my opinion, yes you can do it, but no you probably shouldn't. If anything, consider Power Apps/Automate a supplement to your existing things.
