Please read one of my previous post carefully, as I said, Safari 14 works fine BUT it causes troubles if the following requirements are:
- Safari 14 already installed
- performing any Security updates
-> in that particular conditions (or order), here excessive memory usage happens.
Of course, if you installed Security Update first, then Safari 14, it's fine... until you install a new Security Update! (because you'll perform this new Security Update with Safari 14 already installed)
[edit] that's why re-installing Mojave (which brings back Safari 12.something), then Security Update (004, forget the 005 for now) is fine.
Hope I'm clear enough though