Great question. IMO, Heracross is probably one of the best examples. They had practically zero good (or even average) STAB moves before Kanto, yet reliable access to Brick Break pre-Whitney and Close Combat pre-E4 (most likely pre-Clair) goes a long way in beefing them up; natural Aerial Ace and TM Dig are neat too! Their GSC counterpart had zero good Fighting-type or Bug-type STABs till like Red, unless you got fortunate with Hidden Power.
Surskit, you are correct on. Surskit also in general got a nice buff due to being finally able to exploit their not-awful Special Attack for STAB.
To be frank, Charmander arguably became a bit worse in FRLG against Brock, given that he can now use Rock-type attacks on you. I literally lost Charmander to him in two or more Hardcore Nuzlockes by virtue of that fact (and not having a Mankey). Metal Claw ultimately really lacks power, and you could defeat RB Onix fairly easily by just exploiting Bide carefully with Growl and using Ember; he had no Rock-type attacks for you. On the other hand, much easier access to Flamethrower via Game Corner and level-up, coupled with a much better Special Attack, is fantastic.
As for Cyndaquil, I disagree there too; you can get Solarbeam, Sunny Day (IIRC), and Fire Blast all in the Goldenrod Mart. Fire Blast is vastly stronger than Fire Punch (even if the accuracy and low PP can be annoying outside of boss fights and, at times, inside), and reliable early access makes Cynda a lot better IMO, especially given that only Typhlosion could actually use Thunderpunch and Fire Punch. GSC Cynda is good, but a bit overrated IMO.
Abra, I agree on, more or less.