I use a Fairphone 4 on channel 24.04/daily.
When I did my research and asked the same questions you have, I came to the conclusion that USB3.0 display port with a wired connection to HDMI monitor was superior to any other USB2.0 device which has to use some kind of other technology to connect to another screen, but can connect a USB keyboard and a mouse.
From my USBC3.0 display out decision I figured out that Fairphone 4 or 5 would be suitable although very expensive.
The next question was if 6GB RAM would be enough (yes it is enough), or if I need 8GB RAM.
I have started with the banking app world with Waydroid. It looks promising as it seems that banking apps have changed their restrictions and are ok with running on waydroid. 4.9 GiB RAM is being used while having waydroid and all other kinds of apps open, but swapping of 900 MiB has started to appear. sysctl vm.swappiness equals 100 (=max) so I lowered it to 1 to see what happens. So far no crashes. It still appears to be enough with 6GB RAM to run waydroid and apps.
Waydroid in desktop mode on a 24 inch monitor 1920x1080 needs completely different settings than in native portrait mode 6.5 inches and I haven't figured out how to make waydroid look good in both modes. I chose landscape mode and seem to be stuck in those settings even after disconnecting the HDMI monitor, so obviously I made the wrong decision adapting waydroid to landscape desktop mode 1920x1080. For banking you would obviously want to use LibreOffice Calc next to waydroid side by side and copy-paste between waydroid and LibreOffice. Waydroid seems to require full screen landscape in desktop mode with huge text, although it would probably be ok with portrait mode waydroid on half of the monitor screen. Waydroid may need some fine tuning on Fairphone 4. It looks promising. I think it is more practical to use a Linux Desktop for banking stuff at the moment if you need to use spreadsheets at the same time.
Fairphone 5 is the only one still being sold. Fairphone 4 is much harder to find. Usually there are sales on Fairphone 5 during times like Black Friday, but there are no units in stock to be sold over the counter, only online sales.
There is a lot of writing about shifting customer support, lack of spare parts like extra batteries, extra USBC-ports and price point, which made me hesitate a lot.
It didn't get better to see 12GB RAM Pixel 9/10 sell-outs for as little as 270 €, but of course: There is no port!
I landed in that it is probably wiser to wait until a cheap 12-16GB RAM device with USB3 display out is ported.
Fairphone 5 was the only option available at the time so it was either to accept it and be ok with the price point or wait until something like Redmi 18C with USB3 arrives and sells out for 100 €.
The frustration is on my part that you have to anticipate which device will most probably be ported in the future and buy it when it sells out, maybe years before it gets ported. You can't easily get such a device later. On top you have this whole locked bootloader unlocking process and downgrading android if you are lucky to own a device which gets ported in the future.
In that sense, it was Fairphone or nothing for me. Learning about repair disasters from this forum, I would think twice to go for Fairphone with what I know today.
If you have 700 € and consider it worth taking a short term risk, go for a new Fairphone, it is definitely good enough for now. If no 700 € exist , I would personally with the knowledge I now have wait until another cheap 12-16 GB RAM device with USB3 that you can still buy appears in the Ubuntu Touch device list. Android 16 is supposed to offer desktop mode, which might push developers of devices to put in USB3. It is also common with 6000mAh batteries now and tablets most often have even more battery power. I am aware of that it might take time until more devices are ported.
I have been looking around for posts how users with USB2 devices solve the display out need without affecting internet connection, WiFi speeds and so on.
WiFi-7 is another thing that starts popping up and that I personally think is going to become my requirement for future devices.
32 GB RAM seems to become the new norm on desktops, given that many wish to run local ai models which apparently need 20GB RAM to run 14B models on the graphic card.
My Fairphone 4 is good for all I am testing out, which is generally making it into a PC using wifi. I don't use mobile data that much at the moment. My focus is to make the device useful for everything you need in daily life, on any kind of internet connection. Calls can be made via Signal, Linphone, matrix.org, so I don't really need anything more than some sort of internet. Zoom app is another thing to be aware about that does not work on Linux ARM, but probably does work in Waydroid.
Raspberry Pi 5 with 16GB would probably be another cheap PC that fits inside your pocket, although it's wifi is reported to drop speed drastically once you move away a couple of meters from the WiFi-5 access point. It does lack a good form factor with the extra board needed for SSD hard drive to get better speed, so it is not really an option to carry around. It's ai capabilities are not super either.
Which is why I went for a Fairphone and am trying to get all what I need to work properly on it. I think it is the only option right now, which unfortunately makes me who want USB3 become more dependent on one supplier who can do what they want. I don't like that, but it is what it is right now.
Overall I am more than ok with my Fairphone 4. It is not bad at all, but it is an expensive toy. Maybe I was lucky to get a good unit too. It has not broken yet. You still need to be tech savvy to get everything to work the way you wish in Fairphone 4. Not much comes ready out of the box on Fairphone 4 compared to a ten year old computer with latest Linux Desktop. I would not give Ubuntu Touch to others today to use even though they already adopted Linux desktops over Windows and Apples. It is in my opinion still too complex for a newbie regular user who prefers Android and such that has been around longer. I fear I would become a constant PC support for others if I promoted Ubuntu Touch to them today. 2-3 years from now I think Ubuntu Touch will be much better for the large crowds who are not tech enthusiasts. For me personally Ubuntu Touch is good enough already today for most things I need, including python3 scripting, API calls, JSON-data and LibreOffice. Copy-paste issues in Libertine, scrolling with mouse and screenshots of individual windows and regions like you do on a Linux Desktop is my biggest concern right now. Protonvpn I wish to get working and there seems to be a way with using wireguard (search the forum to find the post). I noticed that the old PS2 mouses work better than new gaming mouses in desktop mode. Connecting other things to Fairphone 4 is for me more important than I thought previously. A couple of days ago I was able to mount a USB pendrive, today I didn't succeed to mount an external USB storage.
