[Nokia 3.1 Questions & Answers] araxhiel:
Well... I have managed to (somewhat) solve the issue with keeping the apps running on the background, so I'll share which my experience was, so you can check through it and see if it fits your needs and/or it helps you.

IT MUST BE NOTICED that some of the described stuff is because my personal needs, but nonetheless it can bring some ideas about what could work for you (or anyone else).

First of all, according to a discussion here on XDA there are a couple of evenwell apps that can be disabled and/or deleted from the device. I found that thread a little too late after I've done some of the following steps, but I wanted to point it out just in case you can find something useful.

So, as we have learn on https://dontkillmyapp.com/nokia (DKMA) there are two probable culprits regarding the killing of the apps:

The Powersave App

The Duraspeed process/daemon

Powersave/Evenwell Apps

For this ones I followed the given instructions on the DKMA page to no avail, as they worked only for a certain time and then the background processes started to get killed after a while.

So, after digging on the Nokia Forums I found this discussion and specifically this comment where an user when full ballistic and created a one-line script to delete/uninstall all evenwell applications (this needs to be run on the ADB shell). I must add that after some "trial-error" loops I noticed that sometimes there were some background processes running, so I done the following:

This is for doing a Force Stop to every Evenwell process. I would suggest to running it a couple of times, just in case there are some processes that refuse to be stopped:

for crap in $(pm list packages | grep even | cut -d ":" -f2) ; do am force-stop $crap; done

This is for Disabling every Evenwell process for the current user. Same additional suggestion as above:

for crap in $(pm list packages | grep even | cut -d ":" -f2) ; do pm disable-user $crap; done

This is for Uninstalling every Evenwell process. Same suggestion as above. Please note that this uninstalls the apps ONLY for the current user, that means, only for you (hence the [--user 0] parameter). As rooting is not possible yet, is not possible to get rid of these apps at system level:

for crap in $(pm list packages | grep even | cut -d ":" -f2) ; do pm uninstall --user 0 $crap; done

What those commands do is, first get all the packages that have "even" on their name (you can go and use the full name "evenwell", I left it as it is because I performed this steps after a factory reset), and for each one of them it performs the indicated operation (disabling, force stopping, uninstalling).

After that, I read somewhere else (can't recall exactly were) that it could be useful to clear the cache of Google Services and Google Play Store, and reboot the device afterwards.

Those steps solved most of the issues, as now the applications weren't killed after the screen goes black, or the device is idle. But, after a couple of hours (~3 hours) I still had some issues as the processes were still being killed, so that's when I went to the next item.

Duraspeed

As we know, this is a major annoyance as this thing (service/daemon) is impossible to uninstall, and it's very annoying to disable. What was most annoying is that my device (TA-1074) it wasn't listed on DKMA as a device having that thing, but it seemed that I had it.

So, this is where the fun starts, because although the instructions posted on DKMA works, it seems (basically) that it only works on the current session of the device, meaning that if the device was rebooted the operation was needed to be performed again. There's a discussion on Github where this is explained more lengthy and in more detail.

To add more fun to the mix, it seems that even if the setting was disabled, the Duraspeed service ignores the current value of that setting while booting the system, only taking notice of it when the value changed, meaning that we need to change the value to something else, and then setting it again to 0 (zero) in order to make a successful disabling of Duraspeed. There's no need to say that doing that on every reboot is a PIA.

Any way, on the thread some users suggested using some automation applications (like Tasker or Macrodroid), but not before granting some permissions to the automation apps, similar to what the guys of Syncthing-Android (fork) suggested (just changing the package name, of course).

But, for me here's the catch: none of the automation apps worked for me. It could be for several reasons, because I needed to setup some other things that I need to run at startup, but, well, YMMV.

Anyway, it seems that the developer of Syncthing-Android has added a routine to its application that makes the change of the setting and therefore disabling the Duraspeed service and that such change is live on production, and I can say that it works as intended.

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After doing all of that, I was able keep the background apps alive doesn't matter if I rebooted my phone or not.

I hope that this could be of any help.

Kind regards.