Every businesses experience will be different for so so many reasons, as are the hurdles, bumps and successes.
On-going support is from the partner, these vary massively, you have already engaged with one, and the key word here is “partner” this is a joint venture, you purchase the software and they advise you how to implement it, but the project and decisions are yours, so there is a percentage of burden of responsibility you have to take, this will be true of your Epicor implementation as well. Step 1 you say your experience has been horrific - why? Understand why and do not make the same mistakes again.
Implemented in brilliant projects and horrific projects and I am the same person, the same constant. If you want horror stories I can give you some, buying the wrong software, no internal project team, no idea of business processes, continual changing requirements, no understanding of the current system, no understanding of what they wanted from the new system, no idea why they were changing, no committment to the project, no management support for the project, no training undertaken, go-live to a pre-determined date with no movement due to changing circumstances, no data cleansing, poor data migration, no established processes prior to live, no established processes post live, the list really is endless.
Note at this point I have not mentioned software, because to be frank it is not relevant. Whilst businesses have made the wrong purchasing decision they can make the right one and still get it wrong.
Software is not a magical panacea it is a tool, an expensive complex tool you need to understand and leverage for the best of your business. Make sure you get the software decision right, the partner decision right, resource it correctly internally and give it the importance it requires whilst managing the entire project. Set realistic timescales and get the buy in from all stake holders, plan, agree processes and train users. Simple really.