This is easy...after you've done it once
First, uninstall Banana, reboot, install Voicemeeter Potato, reboot, then go into the Windows Sound Control Panel, find every instance of a Voicemeeter device and, in the advanced properties, change the sample rate to 48KHz. Do this for both the recording and playback tabs. Also change the sample rates for your USB headset to 48KHz if you can, and your PC speakers if you have any.
In the advanced tab of the USB microphone, turn AGC OFF. Make sure the level is at 100%. Make sure the levels for your headset speakers are also at 100% (at least to start with).
With that chore out of the way, let's just concentrate on making things work with Thetis, first, and leave the online SDR for later.
Make the following channel assignments in Voicemeeter Potato:
Assign the USB headset speakers to Hardware Output A1.
Assign the USB headset microphone to Hardware Input A1 (the leftmost channel).
Assign the PC speakers to Hardware Output A2.
Assign Thetis VAC1 to Voicemeeter VAIO for both input and output (also known as channel B1).
Assign Thetis VAC2 to Voicemeeter AUX VAIO for both input and output (also known as channel B2).
*** NOTE: the naming conventions of the three "virtual" channels in Voicemeeter Potato are HUGELY confusing. They will not appear the same to everyone. The first virtual channel (sixth slider after the first five "hardware" sliders) will not have "Aux" or "VAIO3" in the name. The second will have "Aux" in the name. The third will have "VAIO3" in the name. Just do your best here, it may take a little trial and error.
Assuming you have the assignments correct, now you need to set up "sends", "routes" or "patches", as they are known in the audio mixing world. Those are the buttons next to each slider. They are labeled A1 through A5 to correspond to the first five "hardware" channels, and B1 through B3 to correspond to the three "virtual" channels. I'm going to call these "sends" because they act to "send" audio from one channel to another.
On channel A1 activate the B1 button to send USB mic audio to Thetis.
On channel B1 activate the A1 and A2 buttons to send Thetis RX1 audio to your headphones and speakers.
On channel B2 activate the A1 and A2 buttons to send Thetis RX2 audio to your headphones and speakers.
Use the "master section" A1 and A2 sliders to control speaker and headphone volume.
Use the hardware A1 slider to control microphone gain.
Don't mess with any other sliders, just leave them all at 0dB.
You will be able to use the third virtual channel, Voicemeeter VAIO3, aka B3, to route audio to and from digi mode software if you wish.
You can right click on the channel labels in Voicemeeter to customize them which might help you keep track of things.