Wiring solution for 240V EV charging outlet in garage with long run to breaker box
That sounds like a reasonable plan.  The only thing making me itchy is the number of 50-60A (read: 40-48A continuous) EVSE's on the market.  Given the upfront expense and labor, I'd really rather see you wire this with #6Cu or better, #4Al (Al since it becomes very appropriate at these large sizes and you'll like the price.) 

The only downside of this is you will need a longer junction box to make the splices.  A simple 4-11/16" box won't cut it anymore. 

Technically, you need THWN or preferably THWN-2 rated wire.  However electrical supply houses don't like stocking 2 kinds of wire merely to save a nickel a reel on the price difference between THHN and THWN-2, so almost any wire you buy these days is dual-rated THHN/THWN-2. 



As far as splicing, you're right.  Wire-nuts are going to be a pain at #8.  So I recommend Polaris connectors (lug connectors with a rubber wrapping), and since Polaris is very capable of handling sizes larger than #8, and handles aluminum very effectively, again this reduces the barrier cost for using #4 aluminum in the THWN-2 section. 

Another reason I am trying to sneak up to #4 is that at this size or above, wire marking is allowed.  That means you can simply designate hots, neutral and ground  by marking wire ends with tape, which means you can simply buy 1 spool instead of having to buy 3-4 cut sections.   Speaking of that, there is never any need to distinguish hot phases in a split-phase circuit, so don't swerve out of your way to make one hot red.

The downside of #4 and up is it changes the splice rules from cubic inches to bending-radius. So get a long narrow junction box.  Don't shop for big boxes at big-box stores, they won't have decent selection or price. They also tend to obscenely rip you off on anything but certain commodities they think you'll price-check, that is why they're out of line on by-the-foot THWN-2 (not a commodity, unlike Romex).  Visit a few electrical supply houses and see if you can get trade pricing out of them. Most will give it to you if you reveal yourself to be not an idiot. 



I just saw a Deep Space Nine episode where a customer runs a sexy holo-suite (holodeck) program. The camera reveals the digital companion's legs, waist, and finally face -- the ugly mug of the Ferengi bartender. I thought, "Finally!  A way to express what pulling cable through conduit is like!" 

Cable in conduit isn't illegal - it doesn't need to be. It's a miserable hateful pull which risks damaging the wire.  The irony is, if that's your first conduit pull, you'll never know the difference. You'll just think conduit is a miserable experience.  I use stranded THWN-2 wire almost exclusively, and my experience with conduit is so good that I fit conduit even where I don't need to.  

It really helps a cable-in-conduit pull to make the conduit quite large.  In the case of oblong cables like UF, that's mandatory.  For very sensible reasons, a wide flat cable like UF (or any non-round cable) is treated as a round wire of the largest dimension.  This requires larger conduit even for 1 circuit.  For 2 circuits, the sizes get truly wasteful, and that's a shame because conduit can support up to 4 circuits, after all.

I once did a calc on one 6-3UF and one 12-2UF. The conduit size was frightful. 

As far as routing conduit, remember there is such a thing as a conduit body - typically found in the form of a corner. This allows conduit to make sharp 90s and provide a pulling point, at the cost that the cover must remain accessible.  These come in LL, LB, LR or inline.  Other than that, aim for no more than 1 or 2 90 degrees between access points. Up to 4 is legal, but starts to require serious pulling tools, and for a novice, that often means project failure. 

Assembling conduit sections 1 stick at a time over the wire/cable is specifically forbidden in Code.  That tells you somebody managed to cause a warehouse fire by doing this.  Generally if a pull is that hard, that is God and NFPA's way of saying "don't do that pull" or "more conduit bodies".