If I'm not mistaken, your 2011 iMac only has USB 2.0 ports, not the faster USB3.0. For most interactive uses, there is a "fast enough" disk speed and USB 2 isn't fast enough, while USB 3 is (and USB 3.1 gen 2 beats all but the fastest internal drives, whether HDD or SSD).
The external SSD recommendation you read about is partly a cost saving move and partly due to the current size limits for affordable SSD. Apple is charging a very high increment for the 1 Tb internal, so it's far more cost effective to put in a smaller internal unit and then expand to an external. Since the new machines have the very fast USB 3.1 (as well as Thunderbolt 3, which is also very fast but external units are more expensive), you won't see a speed difference internal vs external for most uses. And, if you need more than 1 Tb storage, you have to either go with HDD (slow) or add an external SSD (fast), since >1Tb SSD's are rare and very expensive today. A properly chosen external hard drive won't wear out any faster than an internal (i.e. you pick a drive meant for always-on use, not a backup grade drive), and of course SSD doesn't need to spin-down to save power or wear.
On your machine I think you'll see a significant delay using an external drive. If you can arrange to move stuff you very rarely access to the external, that might be fine. If splitting your libraries is too inconvenient, then you either have to put a larger HDD into your iMac, or buy a new(er) one.