this router is difficult to maintain
Would using the mainline of OpenWRT help? Or is it more of a bash script/config that you’re struggling with? Package dependencies?
maintained with a similar feel to a standard Linux/BSD system
That is quite generic and depends on what you’re used to I guess.
Some people deploy their configs with Ansible/Nix, some others just copy paste scripts from their dotfiles. 
Given these requirements, should I set up OPNSense or pfSense myself?
OPNSense is OpenBSD, so it will be quite a bit different from your current Debian.
Also, OpenWRT and OPNSense are also quite different as a whole.
Can OpenWRT be maintained with a similar feel to a standard Linux server?
No expert (yet) but the docs are EXTENSIVE and there is definitely a way to back it up or automate the whole process with specific build images + import all the packages you need with the official package manager. 
Also, I feel like this question doesn’t specifically match a router recommendation. 
I went a big rabbit hole as of lately and I’d say the opposite actually: some of them are no-brainers and are quite good given the OpenWRT forum. 
OpenWrt One seems very meh based on specs + price
This is where I’d tend to disagree. If you don’t have huge needs like SFP28 or alike, that 1GB of RAM + CPU is definitely more than enough. The current situation is that the R4-pro will probably be one of the first routers to fully embrace a stable SFP+ with a stable WiFi 7 milestone.
Meanwhile, it is still very early days on a lot of aspects.
One of them, being WiFi 7 as a whole. Looking at the threads, it is still very experimental and hacky to have something decent working. That’s also without talking about the shielding, casing situation etc etc…overall if you don’t own a 3d printer and are not already very knowledgeable with OpenWRT, it sounds like a big leap of faith to commit to it now.
The stocks aren’t also super full and the main seller is still making adjustments to the heatsink, they are still not running mainline OpenWRT either, and all of this is like ~2 weeks fresh news as of today. While the whole package is about 300€+ for the basic kit of having something potentially fully broken.
TLDR: the R4 Pro is very early adopter, but so is WiFi 7 tbh. Without even talking about the fact that you might not even get any clients for it or that it might perform just worse than a proper clean WiFi 6 setup.
So yes, the OpenWRT Two might be similar to the R4 Pro (or at least inspired from it), but it’s not there yet and that is probably why the release is delayed too.
If you really want to upgrade, there is probably no open source hardware available indeed but it might also just be marketing material anyway. 
I think, it’s definitely the most Apple-y as in very popular, easy to use and powerful as a whole but I’m sure there are better alternatives in terms of raw power for the same price.
Most people that buy Ubiquiti are probably fine being locked in their ecosystem and commit to buying all of their networking gear from them I assume. 
I’d be looking at Protectli
I was very tempted a few months ago too but then saw that it’s pretty much over-priced chinese boxes that could be bought for far cheaper and probably overkill if you don’t plan to have a NAS, Proxmox etc…running on it. Which is (AFAIK) quite a bad idea to mix & match networking with everything else.
Not sure I fully understand why it’s so expensive, probably IDPS and other firewall-specific things 
Yes, exactly for why it’s a flop.
But for the cash grab: I would tend to say that it’s more of a “product released too soon”. WiFi 7 is just not mature enough from what I read.
From what I can see, only the Omnia (non NG) is officially supported in mainline OpenWRT as of today
It looks cool! 
Few random notes from me:
- passive cooling? damn, the throughput and those SFP+ ports are heating up quite a lot if you push them with copper (from what I understood of 10 Gb setups)
- I am not sure how Qualcomm is performing but MTK is still the official go-to recommendation
- from what I read, SoCs like MTK are meant to offload some work with their hardware rather than “brute force” it with raw power by using x86/arm, but I guess that if heat/consumption is less of a prio in comparison to raw performance, it might be cool!

- not sure about the CPU, but the other specs look comparable to the R4 Pro (I’m no expert but there are some matches on both sides haha)
- only 2 GB of RAM tho, but plenty overkill if not running your entire homelab on it I assume
- the whole Turris thing looks quite cool
- company looks nice, not shipping 20 products at the same time but looks like they are focused on a few good products
- also the second
company that I know doing routers now, cool to see variety!