I never found the second runs of the GnG games to be too bad, since at that point you already found your 1up/continue farming spots, the invisible triggers to make chests spawns, and which sections of maps to speedrun through to avoid getting overwhelmed by enemy spawns. Just mainly had to keep an eye out for any swapped enemy spawns, since there were a couple extra Red Arremers in spots.
Only thing I didn't know was just how atrocious the slowdowns were on SNES SGnG. I played the entire series on PSP (thanks Capcom for portin' them), and the emulated copy of SGnG in that package had all the slowdowns removed, made it much more playable/reliable. The amount of times I fuck up my double-jumps on SNES Classic because the game decides mid-jump to spawn 1-2 enemies and introduce slowdown is aggravating.
Older you must hate younger you since I recall that final boss had an extremely simple pattern.
Spoiler: show
I'm not sure if I want to beat it again on SNES Classic with all the slowdown, to do the whole "beat every game on this device" self-challenge. Been working on Contra 3 here otherwise, and all I have to say is fuck the Mode7 stages. Shit is too zoomed in, it makes me motion sick for whatever reason. The boss encounters in them is just constant circle strafing waiting on RNG to dictate you get a safe opening to shoot at them.