I admit that a large capacity RAID is in fact helpful even for hobbyist use-case, the media can approach unmanageable size where placing them all over different volumes is just not very practical. I guess my point was more towards a general consumer use case where a Drobo is most likely overkill financially, but in gsusser's case apparently he can afford it and has used large enclosures before, so he is a minority I suppose.
Now back to the question:
I am in a similar situation right now, as a multimedia artist I got way too many drives of past job data mostly Adobe CC projects, with occasional video editing and audio, but mostly an exponentially growing Lightroom library with thousands of RAWs. With a recent purchase of the iMac 2017, I thought it's a good time shifting to a Thunderbolt 3, 4/5bay enclosure with stripped, and/or parity. After some research it seems the options are still limited, the tech is there but not been long, more importantly the market is still slow in picking TB3 up. Drobo 5D3 is one of the few solutions that has no compromise, but it comes with an upfront price.
For my case, the data transfer speed is important, I am currently using combinations of TB2 and TB1 dual drive RAID-0's, some times they suffice but other times they are the bottleneck. The internal SSDs of my iMac and MBP's are fast enough for current active projects, but it is a hassle to move stuff in and out of the internal drive, it also messes up my timed back up schedules or I need special attention / rules concerning where exactly the latest files are on.
In that light I am seriously considering the Drobo 5D3 myself. Having TB3 or not is one of the most asked questions out there, whether the extra bandwidth is practically needed for HDD arrays. Case in point: no it is not even close to saturating TB2 speed let alone TB3. A 4/5 bay enclosure needs to use SSDs to get like half the potential pipeline. But in practice, having a TB3 enclosure means I am future proof, and as a starting point of a downstream TB3 chain it compromises nothing, compared to say a USB-C3.1 or TB2 enclosure in place, so whatever monitor or another drive down the chain is still getting mostly fully covered.
gsusser's case with static media has some quite different criteria I suppose. For one I think parity for large videos is a waste. Maybe a much slower, off desk backup done every few days/weeks is adequate already. Having stripped array helps in speed and the benefit of using just a single volume for ease, but in turn takes away data safety. So the logical next step is RAID5, which does a bit of both, but then the question shifts to whether or not such investment is worth it financially, which only you can answer.
The other posters have pointed out as well, it is relatively cheap to get USB3 drives for stop gap media moving. Since you already ordered the Drobo anyway, may as well wait for it to come, throw in some blank drives (if you have), or buy some blank drives (which can be cheap can be expensive), and start rebuilding that way. Drobo has the ability to "grow as you build" which makes moving stuff a little easier, you can say empty one drive from your StarTech to the stop gap USB3 drive, then yank out that one drive from the old array and put it into the Drobo, start building from there, and repeat.
