Trello.com Trello is free for unlimited users. Period.
You almost definitely don't need "Sub-cards". Use the checklists instead, or if you REALLY need sub-cards, don't have a parent sub-card. Just name the tickets something like "Epic - Story A" or "Story - task Z" or whatever.
Another idea is to create two boards (did I mention you can have unlimited boards for free too?). One for your epics and one for your stories. Call one your product management board and the other your sprint board, or whatever you like.
I'm not sure what you need different roles for - but, people aren't crazy - they know their job. As a startup if you already have problems getting people to not do crazy things (Where you need to restrict their permissions) you have much much bigger issues.
The point is that you need a SMALL tool to help you track stuff. Not a super rigid tool that makes you work in a super specific way. As a new (I assume?) startup, you should let your process grow into a tool. Don't beef up your process to fit a tool.
