Home DIY

I wouldn't go for biscuit joints for two reasons:

1. You really need the T&G because it runs the full length of the planks and will provide much-needed stability to prevent warping. It can also be glued along its whole length. (But, a tip here, is paint the tongues first with at least one coat on the visible outside face so when it all contracts due to weather you don't expose bare wood.)
   
2. Biscuit joints are, to a chap like yourself who goes for the best available, very much the poor relation to Festool's domino system. Still no good for this job, but for other carpentry you'll want Domino and in the end you'll wind up buying into that. The correct (read: perfectly good enough and very cheap) gateway drug while you're developing your carpentry skills is dowels and a Kreg dowelling jig. That is still what I use.)
   
   

A hand-held router is all I have at the mo, and somewhere upthread is a lovely oak and leather desk I made with routed fingerpulls, so you can do a great job with the cheapest of them. But they do like to tip. You need a lot of patience and to take great care. If you're using hardwood it's a lot more difficult not to end up with burned wood, gouges etc and hardwood is a lot more expensive.
You can buy or make cheap tables for them, but a garage door is a big thing. I like Aaron's suggestion of making a custom baseplate for the job. I think that may work as well as a table for cutting grooves and tongues, but for making thin fenestrations I think a table may be the only way.

Floating tenons may work very well. Good shout.