Brand new install of Ubuntu 1804, latest Boinc - 7.16.16 on an FX6300

You probably got BOINC 7.16.16 from Locutus of Borg. I found out on QuChemPedIA that it does not include all the required libraries for that project.
I uninstalled it, and went back to the standard apt version using "sudo apt install boinc". You get BOINC 7.9.3 on Ubuntu 18.04, which works for me.

/tmp would be the place to check. If the message is correct you somehow got it mounted ro. Or the message could be misleading, in that case it could mean you can't write there for other reasons.

Come on, you can't seriously compare FAH to BOINC. The functionality of the FAH client is very limited. That's not negative, it does what it's designed to do. But what is that really? Get a task, finish it, return it, get another one. No decisions to make. And you take that as an example of being smart? BOINC is much more complex than that. Different projects, different applications, work cache, deadlines, resource shares. People use all that to their liking. And then they're annoyed when a machine can't do it on its own or makes decisions they don't approve of. Set up BOINC as simple as FAH. Eliminate the cache, run a single project, and leave it alone. Then you are a big step closer to what FAH does and BOINC will do it well enough. You can carefully add more complexity but remember YOU are responsible for providing the right conditions.

Yes, and that is exactly the reason why BOINC isn't working very well.
Because the scientists need some sort of standard to work towards, like with FAH.
Boinc Mgr won't be able to learn anything from FAH in terms of WU queuing, however they can by looking how well the WUs are optimized for the hardware it's running at.
In my opnion, scientists need a standardized condition, and from that condition each client can individually adjust.

The way things currently are, is that one project aims towards very small WUs, that don't work well on fast hardware; while others run the GPU to 70 or 80% load, while even others are using the full 100% load.
It's a topic more suited for the boinc forums, but the whole boinc statistics, boinc setup, boinc BAM, is a spagetti of all kinds of systems working against one another...
Instead of editing the cc and app_config files, they should have at least some sort of adjustment within the GUI for those things.
Like, use only 1 GPU; Don't use CPU; or set CPU and/or GPU values to for instance 0.5 or 0.33.

Apparently you are not aware that not all projects yield deterministic results. That is, there is some randomness.
That is one of the reasons they have validity checks. Why do you think they send it out to two computers for comparison?
You might as well complain that they are wasting half their processing power if you like. Go where you want.

I think one problem is your idea of how BOINC should work. BOINC is designed to work on its own and make its own decisions. You seem to wish to be always in control, in detail. But babysitting, micromanagement and sudden changes just make things more difficult for the system. Set up the basic guidelines, avoid restrictions and leave it alone, that's how BOINC works best. It may be boring and it takes patience but it works fine.
My cache settings are the same as yours, 1.0+0.1 days, and I added MLC with 10% resource share a few days ago. No interference since and all is well. The client doesn't fetch more tasks than it can handle and does work at will. Turnaround times are approaching one day and will hopefully settle there. After that I may re-evaluate.
Down the thread you ask for more controls to be made available. IMO that's not the way to go. I wouldn't mind if those controls existed but personally I wouldn't use them.

I think what Jim1348 mentioned is a different issue. I've seen it occasionally but not often enough to develop a theory. It seems to happen with new projects or new applications. BOINC then fetches the same amount of work again and again, as fast as possible and apparently without end. No New Tasks will bring it to its senses but by then you'll have far more than you can handle. Surely you would notice if that happened.

I have a Pi4, and while its not my top priority, I should be able to get at least a test app for arm32/arm64 relatively soon. Only time will tell about performance.

What distribution?