$f(1 \ 000)+1$ works quite well with PFGW:
8273631704085440....0000000000000001 is composite: RES64: [BEDCCADA556094C9] (221.6781s+0.0279s).
For $f(n) \pm 1$ with $n \le 1 \ 000$ there are no more PRP's to be announced.
8273631704085440....0000000000000001 is composite: RES64: [BEDCCADA556094C9] (221.6781s+0.0279s).
For $f(n) \pm 1$ with $n \le 1 \ 000$ there are no more PRP's to be announced.