Does a pitcher need a dominant 1st pitch to be successful?
Can you get away with 60's splits and 60's first pitch?
Whats the lowest 1st pitch stat that is acceptable?

While there are always exceptions, a good guideline is that mediocre splits need to be offset by really good pitches.

60's splits are probably a bit above mediocre, depending on whether you're talking about high 60's or low 60's. But a P1 in the 60's is definitely mediocre. You would ideally want to look for a P1 in the 80's.

But what you want and what you can get are often two different things.

I got this guy - Cy Walker. My team sucks, so his wins are low, but since Season 36, he's been very good. 63 P1.

I dont really look at guys who dont have good pitches. I find that guys with pitches in the 60's tend to be pretty mediocre regardless of splits. Gotta have at least 1 pitch above 75

Don't use this as word of God (my home park is SD)

but this guy is mediocre right across the board.
https://www.whatifsports.com/HBD/Pages/Popups/PlayerStats.aspx?pid=4540070

some years he has been very good, others he has been very bad. I usually use him as an LR or spot starter.

successful is a relative term. Should a guy with splits in the 60's and P1-4 of 68-65-58-53 be your #1,2 or 3 probably not but would he be a serviceable #4 or 5. Sure. There are other factors like control, GB/FB even the much maligned velocity which factor into it.

I've got a RP right now, given up 30 hits in 27 innings. All his numbers are acceptable except that 12 of those hits have been HRs.

He's got 80 Control, 63/71 splits, 39 GB/FB, 72 P1 and 66 P2.

Ive got starters with similar splits but better pitches. I've got other pitchers with worse GB/FB, but none of them give up HRs like this guy. He's just an unfortunate combo of ratings, and what jumps out are the worst pitches on the staff.

Cy Walker is an example of a guy with really good spits and 3 high mediocre pitches. You can get guys like this at value and they will help.