I THINK I got it solved with a post, which I will reference right here: (EDIT: It seems like I can’t embed links here, but it was in one of the thread @Takakage referenced)
After checking the link you’ve provided I installed that maxi helper, which told me that I’d need to update my grub.cfg file, which is weird, because I’ve done sudo update-grub and all the good stuff a hundred times already.
As per the link I’ve attached I started with a fresh Grub installation. What makes me think now that my booting issues might have vanished, is the fact that the “Unknown device type” errors I referenced earlier among others are now gone when updating grub. I’m quite confident the issues might have been solved now, but I’ll let you know as soon as I tried another reboot.
That’d be HUGE if it was gone! 
Thank you guys for all your help even if it’s not resolved yet! What a lovely community 
So 
My previous attempt, sadly, didn’t do the trick. Starting with a fresh Grub just had me rebooting twice.
BUT:
I was able to finally solve the issue!! 
On the 2nd reboot I thought it might be a good idea to have a look into the BIOS settings again. Like I tried earlier already, I changed the OS configuration of my MSI motherboard from [UEFI] to [CSM] (Note: I’m still not sure which of those 2 you’d need for Manjaro).
I had a look at the BBS boot priority settings and then I noticed that there’re 2 entries for Manjaro:
The first one was just called “Manjaro” and the 2nd one was something called like “UEFI OS”. Both were enabled, so I disabled the UEFI one et voilá, the double boot cycle issue is gone!!!
As I don’t have any idea about why there were 2 seperate entries and also why I couldn’t see this when my configuration was UEFI and not CSM, it would be huge if somebody with more knowledge could elaborate on this if possible.
For me personally as a beginner I’m just glad about the outcome, that the issue is solved. Still I would like to understand why and how.
Cheers to you guys!
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With CSM enabled the system picks Legacy boot when booting a Manjaro ISO and you could possibly run into issues if you were to have multiboot using grub - as the two modes are incompatible.
To ensure EFI boot disable CSM.
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So it seems to me:
- There is one BIOS-Installation and one UEFI installation intermixed
- Then there is one grub.cfg in the old install and another in the new one …
- And there is one grub in EFI-partition and another somewhere else
I think you will keep having fun in the future with this mess.
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This seems pretty plausible, as I rebooted today the old issues with double boot arose again Is there anything you’d recommend me to do, besides reinstalling Manjaro completely and making sure that all traces of previous installations are wiped? Or you’d say this is the only/cleanest way?
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I’m not particularly savy when it comes to tinkering under the hood myself, so I would advise getting a Stable Branch ISO if you want a relatively stable environment and take it from there. Just my two cents… 
as filesystem
(at least to another device)