Welcome to the forum. (I realize it’s been a few days - I’m slow.)
I’m also a long-time C# dev who’s new to Epicor ERP - I started using C# in ~2007 after almost a decade on VB6/VB.NET, and I started with Epicor just about 13 months ago. I’m not one of the “Epicor Devs” at my employer, though - meaning, the systems I work on are not built in Epicor itself but they interact with it - so for the most part I use Visual Studio 2022 with the occasional foray into VSCode. And obviously a lot of SSMS trying to examine and understand the Epicor ERD at even a basic level. Epicor reminds me a lot of PeopleSoft and the legacy homegrown systems at my previous employer - large systems with decades of functionality layered on organically with multiple styles of implementation likely based on the dev team and best practices for the time. There’s tech debt in the data structures that I suspect is hard to extract and so it just sort of keeps getting layered on top of.
As for this forum, the fine folks here recently helped me through a nasty bramble of API calls to retrieve object data. There’s a lot of good experience and knowledge here and folks are pretty friendly and welcoming to Epicor novices. My only piece of advice is if you’re going to use the provided REST APIs, test them out with Postman and take advantage of the $filter, $select, and $expand properties on the ODATA routes.
I’m not much help with Epicor itself, but if you ever need another set of eyes with C# - design patterns, principles, architecture & solution/system design - don’t hesitate to reach out.