You do not add up the individual breakers to come up with the main breaker. 60 should be plenty. I have a 100a service on a 20 slot main box that feeds 350a to 400a if you add up all the individual breakers. In daily life you never use all that power at once.

Yes, as long as you've go the load issues right, there's no problem running a sub from a sub. My house will have six panels that are effectively subpanels off the main subpanel which is distinct fro the Service disconnect.

The load to be served is well defined at this point. If the OP adds more large equipment that requires power and needs to run more than one piece of equipment at a time (beyond a dust collector and a single tool) he can always upgrade the feed. There are significant economies of scale in purchasing things like 100 A panels instead of 60 A. They make a lot more 100 A panels, and while it is overkill for a 60 A feed it is not in violation of any code in feeding it with less. I have even fed 200 amp panels with 100 amp feeds for large workshops. The ability to have plenty of circuits each on separate breakers has advantages. I would not install a main lug panel unless it was adjacent to the panel feeding it. It is worth the few $$ for a main breaker in the sub. The only time you need to wander over to the main is if you are working on the feed to the sub. Work in the sub can be done with the main at the sub turned off, leaving opnly the input to the main breaker hot.

It makes no difference. An AFCI works just fine in any whatever panel is feeding the branch circuit.