EBS 12.5 doesn't exist — you mean 12.2.5 I guess.
Q1 – RHEL 7→8: Rapid Clone works, but expect relinking (DB + EBS stack). More effort than RHEL 7→ RHEL 7.
Q2 – OEL 8: Almost same as RHEL 8, no difference. Prereqs: OS packages, AD/TXK patches, relink after clone.
Q3 – Recommendation: Keep RHEL 7.9→7.9. Less risk, tight window. Upgrade OS separately later. An in-place upgrade may be done. ->
https://blogs.oracle.com/ebstech/inplace-upgrade-from-oracle-linux-7-to-8-now-certified-for-ebs-122extra ref(s):
Oracle E-Business Suite Installation and Upgrade Notes Release 12 (12.2) for Linux x86-64 (MOS Note 1330701.1)
Upgrade Oracle Linux with Leapp