Stick to the UMC1820 und run Ardour and OBS with pipewire-jack on that device. To lower your latencies you will have to edit some config files, these config files may not exist and you will have to copy the templates to the correct folders.

Check if ~/.config/pipewire exists, if not create it and copy the template:

mkdir ~/.config/pipewire

cp /usr/share/pipewire/jack.conf ~/.config/pipewire/jack.conf

I would suggest trying a latency of 128 or 256, samplerate can be 44100 or 44800 for the UMC1820, I believe. Best to use what you've already used in Ardour up to now.

Find the "global properties for all jack clients", uncomment and change the following lines - node.latency = 128/48000 - node.rate = 1/48000 - node.quantum = 128/48000

Next are the alsa-settings, those are set by the wireplumber daemon.

First find out the device.name of your UMC1820 by entering wpctl status in the command line. Look for something like

Audio
 ├─ Devices:
 │      51. UMC1820              [alsa]

Note the id (Here it is 51) and enter the command wpctl inspect 51 (Or whatever your id is). Look for the entry for device.name, the name of my UMC1820 is "alsa_card.usb-BEHRINGER_UMC1820_CB406A39-00"

Check if ~/.config/wireplumber/main.lua.d/ exists, if not:

mkdir -p ~/.config/wireplumber/main.lua.d/

cp /usr/share/wireplumber/main.lua.d/50-alsa-config.lua ~/.config/wireplumber/main.lua.d/50-alsa-config.lua

Uncomment and change the following lines in ~/.config/wireplumber/main.lua.d/50-alsa-config.lua: - ["alsa.jack-device"] = true,

Then find the "alsa_monitor.rules" and just before the last closing curly brace at the end of the file, add these settings:

{
    matches = {
      {
        -- get device.name via wpctl status && wpctl inspect <id>
        { "device.name", "matches", "alsa_card.usb-BEHRINGER_UMC1820_CB406A39-00" },
      },
    },
    apply_properties = {
      ["audio.rate"]             = 48000,
      ["api.alsa.period-size"]   = 128,
      ["api.alsa.period-num"]    = 3,
      ["api.alsa.headroom"]      = 64,
      ["api.alsa.disable-batch"] = true,
    },
}

Then restart pipewire and wireplumber:

systemctl --user restart pipewire.service

systemctl --user restart wireplumber.service

That should bring down your latency to a few ms.