Glad to hear you are thinking of moving to Austin for retirement. There are really so many good things about Austin it is the main reason 130 people are moving here every day. Now that is also the one bad thing is traffic can be a bit of a headache but if you avoid the typical rush hour times there is absolutely no problem.
As for 78721, you will be happy to know it is very close to #1 rated BBQ place in all the country. That is not self proclaimed either. Franklin's BBQ is the rated the best in the country and I can tell you I have been to 100 top BBQ joints across the US and none beats Franklins. You will also be near one of the best Mexican restaurants in Austin in Juan in a Million.
78721 is also a great area because you are not far from downtown, UT, or the airport but not paying high real estate prices either. There are currently just 4 places for rent and one apartment complex in 78721. I am happy to email you those and see what you think.
Let me know how I can help. My services are absolutely free of charge.
Dan Burstain, Realtor
JB Goodwin Realtors, serving Austin for 40 years