Can I put the sub-panel for my attached garage in the attic?

Yes: Accessibility and working space.

You need a zone 30" wide x 36" tall x 78" high in front of the panel, so the electrician can stand and work on the panel. This space must be clear at all times; it cannot be blocked by anything. Ingress to the space must also be clear.

Why is this required? Because quite often, when a person is being injured or shocked by a machine, the only sure way to stop the injury or electrocution is to shut off the breaker. For that to happen the breaker has to be accessible.

The problem with your flop-down ladder is that invariably, that space it flops down into is blocked by other things. Like in a garage, a car parks there. Can you imagine the absurdity? You're being shocked by a saw, and your partner has to run into the house, find the keys, pull the car out, lower the loft ladder, climb the loft ladder and cut the power? No sale.

The ideal place for a panel is in a threshold or pathway, so that the working space is inherently kept clear because it's a pathway.

Also, go big on your panel. You'll thank us later.

Breaker spaces are worth their weight in gold when you're trying to expand your electrical. It's very frustrating and expensive to find yourself "out of spaces". Yet, spaces are dirt cheap when you are initially installing the panel. Therefore, we recommend you always install a Very Large Panel. Like, absurdly large, like "will never use all of those spaces" large. You'll use more than you think, especially when you have a surplus. The cost is very small compared to total project cost.

There are lots of other places to scrimp (why on earth spend on copper feeder?) but the panel is not one of them.

A 30-space panel is not excessive.

Since you're using a 50A breaker to supply a 60A panel, you know that panels are allowed to have busing that is larger than the supply breaker. So it could also have 125A or 200A busing. There's not a problem there.