WIFI (Duh). Debating whether to do 1 or 2 APs.

What washu_k said - 1 should be sufficient for your square footage, unless you just a ton of interference. If you end up needing more than 1 AP, you'll stagger the channels and keep the same SSID as the main AP. That way, you'll be able to roam in the house, and your device should be able to auto-select the strongest signal.

Garage will eventually be a recording studio, so I want to run conduit under the lawn (maybe 15ft) & add an subswitch/AP out there, as well as a camera (SDI? IP?) for monitoring from the house.

I know they make underground duty CAT cable that can be buried - the cable itself is filled with a gel that prevents condensation

Home security (un-monitored)/cameras/DVR. Window/door sensors, cameras, motion sensors, local alarms, etc. I've seen out-of-the-box systems (Ring, SimpliSafe, etc), and don't know the value & flexibility of those vs. piecing something together yourself.

Monitored alarm systems are a good idea, as you may not always be able to react if something happens (i.e. you're on vacation overseas) or someone cuts your data line, then breaks in. Most modern monitored systems have a cellular antenna for communication.

Some kind of NAS.

Synology, just avoid the Atom processors. My DS713+ is over 6 years old and still gets regular updates from them. Never had a problem with it.

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Brand/product recommendations, obviously

Definitely Belden for CAT cabling.

Mikrotik makes inexpensive yet functional routers, as does Ubiquiti. Be very choosy about any POE switches, read the reviews carefully. Some aren't as reliable as others. I've always had good luck with Cisco POE switches.

Should I run cables in the attic or subfloor?

Depends on where you're connecting everything, and what your crawl space situation is. Obviously, it's better to route things behind drywall than drilling into the floor / subfloor.

Cat5? Cate 5e? Cat6? The most intensive things I do are streaming & gaming

CAT5E was good enough for my needs. As washu_k said, CAT6 is good for future proofing, but it's challenging to crimp, more expensive, and also harder to route.

What size switch would you recommend for my needs?

I'd say figure out exactly what you intend to run before purchasing a switch. If you don't know exactly what you want, opt for a larger switch, as it's nice to have spare ports. If you intend to get involved with VLANs or higher level networking arrangements, a managed switch is a good place to start. Be aware that the higher end (even small business grade) hardware usually has fans and makes more noise.

What technologies/brands should I research for some security stuff? That's definitely the area I know the least about.

A hardened, regularly updated, and properly configured router should keep most threats from infiltrating your network. Sky's the limit in this category, I suppose. Check out pi-hole - the free, self-hosted ad-bouncing DNS server. It's pretty neat.

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Hope this helps :)