Adding a garage subpanel
Go big, Big, BIG on the subpanel

Especially if your main panel is getting full, and having double-stuff breakers in it definitely counts as "getting full".  

We have so many questions on here of "My panel is full. What can I do?" And our answers are expensive and regretful.   But that we could go back in time, to when that panel was being bought, and say "Spend a couple lattes on a bigger panel".  So, we say it now.  Go big.  Really big. 

You're at 7 breakers so far (2 for the heater) + a foreseeable dust collector and EVSE, so 10. I like to finish with all full-size breakers and >50% of spaces unused, so a 20-24 space panel. Disregard "x circuits" as that calls for double-stuff breakers which are no longer viable due to Code changes. (they don't mention that in the marketing materials :b)

Don't let panel bus rating hold you back: An "R" rated tire is only rated for 85 mph, would you put that on your car? Not willingly!  So it's not a win to get a panel with 100A busing. So don't think twice about a main-lug breaker that's 225A/24 space if you can find a happy price on it.   

The main breaker doesn't buy you anything (sometimes main-breaker panels come bundled with some bonus breakers, so that can be a savings), but the main breaker per se is irrelevant. You can't make the more convenient breaker trip first, because of main vs branch trip curves and Murphy's Law.  Even if you needed a main disconnect switch, the trip value is irrelevant; feel free to feed a 225A subpanel from a 30A feed breaker.  Totally legit. 

Your feeder wire

2/2/2/4 is an unnecessary size, but I understand why it seems correct. 

First, if it's convenient to run conduit from the main to subpanel, and  you want to do that, use THHN/THWN-2 individual wires inside the conduit.  

Don't bother with copper unless it's like a 10 foot run or something.  The lugs the subpanel will be aluminum anyway because aluminum lugs play nicely with both.  So why create a dissimilar metal "problem"?  (isn't actually much of a problem on these fat feeders).  But if you do go copper, #3 will suffice and #8 Cu ground is all you need. 

If you go aluminum (most of us would), you need #1 AWG. 

Loads

You're in very good shape.  Your PC and tools will chomp only 15A of your 100A. Electric garage heaters range from 15A to 50A, but you're definitely covered, even with a 60A sub. But stay at 100A. 

Here's why I don't think 100A in a garage is overkill.  The automotive world is changing so very fast. "Range anxiety" which used to stop the electric car industry dead in its tracks, is being alleviated from both ends: First, consumers are becoming much more aware of their actual driving patterns. And second, Tesla and others are making is making enroute recharging look stupid-easy. I do lots of transcon trips, and Tesla is at the point where I could recharge in the time I already stop for gas, bathroom, and meals. So EVs are happening, so having 50-80A of headroom in a garage panel is great, and can only help resale value.