Phew.
The landscape has changed since I last did this. One thing has not changed. 2.5" drives use 4 pegs to stay in place. I brilliantly forgot to swap them from the ca. 2011 HDD to the new SSD and realized the fact just as I tightened on the last screw on the underside panel of the Mac. Many screw (and curses) later, I re-opened the laptop, extracted the SSD, attached its 4 pegs and re-installed everything. Blissfully, the machine booted with no problem. Then I had to enable TRIM. Years ago I used a freeware app called TRIM Enabler. In 2019, said app is no longer free and arguably malware. Fortunately, there's a command in Terminal that does the same thing: sudo trimforce enable. The above screenshot confirms it's working.
Even with a 2011 i7, going from 4GB of RAM and a slow AF 2.5" 750GB HDD to 16GB of RAM and a modern SSD is like working on an entirely new machine. High Sierra was not meant to run with 4GB.