I fall into the "former 4.0" recently slid (bad year) to 3.5 group (but intending to get back to 4.0....).
@ 56 yrs old I simply don't have the wheels anymore to charge in after a hard 1st (I'm 6'4" - playing since I was 10, so my 1st is still up near or in 3 digits). It simply gets there too fast for me to have enough time to recover and charge.
My preferred serve to follow in - either 1st or 2nd - is a hard slider (sorry I use baseball terms for tennis too) into the body. If I can get my opponent crossed up they're going to set up a sitter for me to pick off as my 1st volley, or if they manage to get more than that on it I still have an easy first volley to set up the kill.
The hard slice serve (into the body) is a very underrated weapon. When it works it's easily as effective as a boomer and much more reliable, as the topspin keeps it in.
I reserve my flat 1st for when I'm comfortably up in the score (40-L or 40-15) and I'm purely going for an ace.
Just my .02¢