First off: I never said anywhere that you'll only be able to one mechanic per evening.
Right now you're so caught up in your belief that Savage is an insurmountable task and that it's hopeless trying to clear it in PF that you're interpreting everything that way.
What I said is that there are some meme mechanics in PF because they tend to hold up groups a bit longer and that I rarely see a PF NOT getting past it within the evening. I never said that it's the ONLY mechanic you'll see within the evening once past it nor have I said that every PF is like that.
You're still making a lot of assumptions about something you haven't actively tried.
Anecdotally I spent 4 tiers (all 3 ShB tiers and the current EW tier) PFing quite a bit and not just reading PF descriptions or checking names and every "helpers welcome" party usually had 2-3 helpers minimum. Sometimes it was only one person still needing the clear and the rest were helpers. The parties don't have "We're a bunch of pros that will help you learn this fight and teach you everything" in their description, they are regular "practice" parties with a "helpers welcome" in the description in addition to things like "bring macro" or "we have markers" or "R1 flex". Perhaps that is the reason why you missed it.
Sometimes people leave and that is normal. PF is a hop on/ hop off thing precisely for people like you that can't set aside a fixed amount of time every week on the same days; some people only have an hour, some people want to progress faster, others notice that they aren't as sure with mechanics as they thought and leave.
You are also free to make your own party, set whichever mechanical requirement you want (e.g. "numbers onwards") and therefor setting the prog speed, welcome helpers and replace people you feel are holding you back. If you're only looking at parties from others you'll have to take what you get and may need to deal with someone that keeps failing something not getting replaced.
As it is endgame content, it's not unreasonable to spent 10 hours and more on a boss, depending on thr turn. P1s and p2s can be cleared within one evening respectively if you have a couple of helpers that know what they're doing; seen that countless times during all tiers I PF'ed. You'll spend longer on p3s because add phase and FoF tends to hold up PF a bit longer. And obviously even longer on p4s as that's the final boss of the tier.
If that is too long for you then yes, I suppose the time spent/ reward is not to your liking - doesn't make it bad, just your preference of generally wanting to get things a little faster than that. But considering it is the 2nd highest difficulty of any content in the entire game, it's reasonable.
I have to ask: are you really concerned about the difficulty increase from Ex to Savage or are you concerned that PF will hold you back because you deem it too unreliable for you to make progress?