As I hinted, I only have really extensive experience with Raspberry Pis, so I will tell it from my Raspberry Pi perspective.
As with all computers, it always depends on the type of usage. Browser and torrent activities are usually very lightweight, except you watch a lot of YouTube (or whatever video platform), then there are definitely limitations.
If you generally are otherwise very lightweight with your PC usage behaviour, I’d say a Pi 4 8GB (or whatever open hardware alternative there is) should be enough. If you are really focused on lightweightness, even the Pi 4 4GB might be enough.
If you are planning to go into cryptocurrencies or other things that are built on top of a blockchain, it might be more difficult. I could not check out quickly, if Qortal is Proof of Work based or not, but if it uses any kind of computation, I suppose an SBC would have its limits, if you aren’t very patient and forgiving. Additionally, even if it has nothing to do with Proof of Work, then you still need CPU time for cryptographic operations, as wallets and other blockchain stuff is pretty much always encrypted. This draws additional CPU power.
“Light editing” is also a very relative term. If you want to edit a 2 minute video, where you only cut out something, I think it should be reasonably practicable. That said, I’m not so sure how viable this might me, if you go beyond a 10 minute video, with a fairly high quality and lots of editing. Not sure.
I don’t know if those having more than 4GB are specifically open hardware, but I know that there are SBC alternatives in general, that easily have 8GB or even 16GB. But, they are extremely expensive. Such a 16GB version can easily cost 200 or 300 bucks, when it’s just an SBC without all the bells and whistles, like a case, cooling, peripherals, etc.
It’s just not open hardware. Aside from that, I personally do not know of any specific or obvious vulnerabilities. One thing that is definitely for sure: it’s a lot safer than the vulnerable Intel CPUs you have thought of.
Additionally, it’s not a “big thing” (literally). So, it might be a big issue, if you have a closed hardware computer with lots of stuff in it, because there is so much room to have something “hidden” in there, that you don’t want. However, with an SBC, that is comparatively tiny, there is barely a chance, that something is “hidden” there. I mean, it’s just not enough space. It’s not impossible, it’s definitely feasible to compromise it, but it’s probably far less likely to happen on such a tiny product than a real full-sized motherboard.
I would say Raspberry Pi hardware is fairly trustable. It’s not the most open and not the most security centric piece of SBC. That is abundantly clear. Still, it’s, in my view, safe enough for normal use, if you aren’t a spy or otherwise a government agent, even if you want to be extra safe.