NOTE: I am, and will be for the next few years, a 1-man-shop. So I don't need network connectivity.

How much support are you going to need?

One option is building a standalone server using these parts

Is 10TB enough? I'm assuming you're looking at RAID1. Seems kinda overkill, and a USB3 SATA cage could do the same job, but for a lot less. If you really want a server, though, using a dedicated RAID card would allow you to get away with a lot more modest hardware.

plus a thunderbolt card not listed.

Don't bother.

• First: What OS are you running? I have no idea what Thunderbolt support is like in Linux, but it's spotty in Windows, and probably non-existant in FreeBSD (so that rules out FreeNAS).
• Second: Thunderbolt bridges either emulate a 10GbE network interface (in the case of two "active" computers) or a storage device (in the case of Target Disk Mode), so you may as well just use a 10GbE NIC and just directly connect the two computers over that.
• Third: You're never going to get near enough speed out of two spinning disks to need Thunderbolt performance. 7,200 RPM disks run about 800MbPS, so at most you're looking at 1.6GbPS, which you can get out of USB3 or 10GbE easy.

• Promise Pegasus3 R6 (24TB 6-bay TB3) $2154

Promise Pegasus are popular choices.

• G-Tech G-SPEED Shuttle XL (18TB 8-bay TB2 with 2 ev Bay adapters) $2000

If you're buying it with the drives then those'll be Hitachi disks in there. They're considered to be some of the most reliable disks on the market, but keep in mind: all drives fail eventually.

• QNAP TS-453BT3-8G-US (4-bay TB3) $950 + drives

If you aren't using the network hookup, or with no plans to use the network hookup ever, then QNAP isn't necessary.

• Drobo 5D3 (5-bay TB3) $684 + drives

Avoid Drobo like the plague. Performance is totally garbage and they use weird proprietary non-RAID tech that makes data recovery by non-Drobo tools very difficult.

• QNAP TS-963X-2G-US (5HD+4SSD-bay only-USB) $660 + drives

• Synology DS1019+ (5-bay only-USB) $650 + drives

Same as the other QNAP option. Synology tends to offer a slightly nicer management interface, but QNAP is a little cheaper. When it comes down to it, they're both basically small Linux servers running mdadm, which has the advantage of being easy to recover in the event your system blows up.

Server or Pre-made: Build the "server" from the parts in the link, or buy from the choices above (or something similar in the $1500-$2200 price range)?

Depends on how much you want to do managing your own hardware and software, or if you'd rather go with a turn-key solution that has support for both?

USB or Thunderbolt: Are the USB options deal breakers (should I only consider Thunderbolt) for both now and future use?

How much bandwidth do you need? USB performance is below other DAS solutions, but depending on how much storage and bandwidth you need it may be sufficient.

RAID configuration: Which RAID configuration should I use, noting the workflow I presented above, and keeping redundancy and speed as top priorities (not longterm storage). Right now I'm between RAID6 and RAID10.

RAID10 will be faster than RAID6, and potentially able to tolerate more disk failures (depending on configuration and luck), but RAID6 will offer significantly more storage capacity than RAID10.

Drive models: In any of the options where I have a choice in which drives to buy, do I need "enterprise drives"? Or are WD Red Pros, WD Blacks, or even Seagate IronWolfs good?

Yes. RAID/NAS disks have modified error recovery firmware. Basically an HDD has a tiny little computer in it, right? It's what actually controls the spinning of the platters and flitting about of the read/write head. One of the things it does is error control. When a spot on the disk can't be read the disk focuses time on reading that block to recover the data and rewrite it to a good block. This is why computers with dying hard disk tend to lock up: they're waiting on this error recovery process to finish.

Trick is this means one disk having issues can hang your whole array. Instead what RAID/NAS disks do is they take a couple really quick stabs at recovering the data, and then say, "nope! It's corrupt! Too bad!" The idea is that it's quicker to recover any lost data from a bad sector using the redundancy data from one of the other disks than it is to focus on conventional error recovery.

Company longevity: Is there any concern that "newer" companies like Drobo and even QNAP might not be around in a few years, so a faulty controller could cost me all my data?

Well that's also why you want to look at standardization. In the case of QNAP and Synology using mdadm for drive management that means, yes, the company could go belly up and your unit could die, but you could plug the disks into any Linux system and recover the data.