"Where subject to damage" is a judgment call
Everything I've read in various code sections seems to just say "where subject to damage," but I can't tell if this area in the unfinished basement is subject to damage or not.
Yeah, that will vary by the particular viewpoint of the AHJ. However if physical protection is required, you don't need to use electrical conduit, but if you do, make sure it actually provides the physical protection. Some types are not adequate. Nobody ever got red-flagged for using EMT.
You don't need a breaker actually
I am working on adding a sub-panel to my main panel. I have 200 amp service, and I am putting in a 175 amp subpanel (using this: HOM2175BB) and running 4/0-4/0-4/0-2/0 AL SER
NEC 310.15(B)(7) says no residential feeder ever needs to be larger than 83% of the SERVICE size. Your service size is 200A. 83% of that is 166A. 4/0AL is 180A wire.
That means 4/0 is adequate for your entire 200A service as well as ANY feeder to any panel running at any ampacity up to service size (200A).
Since you already have a 200A main breaker, you don't need another one. You can simply use a "Subfeed Lug Kit" (a breaker with no breaker) to tap the main panel.
Plus, if I ever decide that I need to upgrade to 400 amp service, hopefully, I can just reroute the extra 4/0 SER cable up to my meter and feed both 200 amp panels from there with minimal extra wiring.
Well, in that case, the 4/0 wire would not be sufficient. Remember your favorable derate comes off 83% of the service size, which will be 332A on a 400A service. Suddenly 4/0 wire is only good for 180A anymore. Easy answer: use 250 kcmil wire here.
to the other side of my basement into a HOM4080M200PCVP panel. This might be overkill,
No, not at all. We wish more people would get 40-space/200A subpanels.
Your new loads need to work in your Load Calculation
You need to follow NEC Article 220 and do a Load Calculation for the current and planned loads on your panel. Not some other method :)
Your service must be sufficient for the loads to be served, which is to say you're not allowed to overload the service based on the Article 220 Load Calc.
You need to do a Load Calc for each panel, but more importantly you need to do it for the whole service. I am concerned you are piling a lot of new load on, the load may not fit given your panels being full, and this may be unnecessary in any case.
60A is a misconception about EV charging
We get this a lot. You have every EV shipping with a travel charger intended to be used at RV parks, so it comes with an RV connector which is 50A. Then you get the Tesla Wall Connector which is dynamically programmable but maxes out at 60A circuit. And seems like everybody thinks they need a 50-60A circuit to have any hope of sensible level 2 charging at home.
This is simply not true.
I think people haven't earnestly thought about the miles they actually drive. Put it this way, a typical car lease grants 12,000 miles a year. Assuming they only drive weekdays, that is 48 miles a day. And that means you're spending 1-2 hours a day behind the wheel, in suburban traffic typical of areas with high EV adoption.
At 48 miles a day, you can replace that with level 1 charging with a regular old wall plug.
If we go to 240V/30A which is half your plan, we're now charging at four times the speed of level 1, 200 miles a day given 13 hours of charge, and it's vanishingly unlikely that you'll ever need more. And if you do somehow have two 300 mile days in a row, you can always hit a DC fast charger for like 10 minutes to close the gap.
Even this 240v/30A is overkill for a single car. You can get by with 240V/20A (2.6x Level 1). I'm mentioning 30A because you plan two cars.
With two cars, more magic happens
It's easy to get EVSE's that coordinate between each other. So they are together, sharing a single current allocation. Tesla Wall Connector v2 and v3 have this Power Sharing built-in, and other companies market it under names such as Share2. When one car is attached, that car gets all of it. When two are attached, they dynamically split. And when you have two cars, two funny things happen.
First, it is vanishingly unlikely that both cars need big charge at once. You're likely to have 170 miles and 30 miles. That's just the kind of thing that Power Sharing is really good at. Plug in both cars and walk away: They split 30A for a few hours until the 30-mile car is finished, then the other car gets all 30A. "That was easy".
But second, cars tend to arrive staggered. So your functional charging time is from when the first car arrives to when the last car leaves. That means you actually get longer, and get to charge more overall miles.
And then we get rid of the service upgrade.
EVSE's latest trick is to put a clamp meter (Current Transformer) on the house's service wires. The fact is, house draw is rather spiky. It's big bunches of it when the dryer is on, cooking is being done, water heater is cycled on or A/C is cycled on. But most of the time, the house is quiescent - drawing a piddling amount - only an amp or two. (houses average about 4 amps or 1.0 kW, so obviously they're well under 4 amps most of the time.)
By sensing amps on the service wires, the EVSE can know when heavy appliances are running, and simply slow down charge rate to stay within service limits. Since this isn't much of the time, it has little effect on practical charge rates -- but it has a HUGE effect on the need for service upgrades or rather, the lack.
This tech is far cheaper than a service upgrade.
This can combine nicely with someone's desire to have a high-amp charger. If you want to have dual EVSE's sharing 100A *and it has the sense clamps on the service wires, you can do that right now on the service you have.