Are there other protocols for enforcing AAA besides RADIUS?
I am looking for a simpler method of implementing AAA (Authentication, Authorization and Accounting) for approximately 20 users on an IEEE 802.11 access point.
As this indicates you are talking about WPA2-Enterprise, the short answer is no. 802.11 uses 802.1X authentication for this purpose which makes use of EAP authentication methods and a RADIUS server. There is no other AAA solution that will work.
RADIUS works well but limiting the amount and duration of sessions is really messy and tedious to set up for a small set of users.
This seems to be the actual heart of your question but you don't actually define the problems you are having or why you may need to limit amount/duration of user sessions. Product recommendations are off topic here, but there are many different implementations of RADIUS (as well as tools to manage RADIUS servers) and they all have their strengths/weaknesses.