This isn’t how suspensions were initially intended to be used, but it’s a common practice for membership based Discourse sites. Other than that a suspended user cannot be promoted to a higher trust level during the time they are suspended, I don’t think there are any side effects to suspending users.
One possible issue is that staff members will see link to a record of the user’s previous suspensions on the user’s profile page:
That isn’t displayed to regular users though.
I think it’s safe for you to keep using suspensions as a way of dealing with expired memberships.