Weird fluctuations in MIDI messages (multiples USBs)

[BandGeek32]

To be more specific, I am using 3 USB controllers in FL Studio for a sort of "organ" setup.
1) A generic 88 key MIDI keyboard
2) A 12Step foot controller (yes, the one with the rubber keys)
3) An OnStage Expression Pedal connected through a 1/4 to USB converter (where I got the converter from: https://www.audiofront.net/MIDIExpression.php)

What I aimed to do is control volume or expression while also using the foot controller as a bass module and playing the main keyboard. This seems to work fine, except the MIDI value from the expression pedal begins to fluctuate (from what I can tell, a range of ~5 in both directions), which doesn't sound very good when using the pedal for volume.
With my current setup, I can only use a USB splitter to simultaneously connect all 3 devices, having only 2 USB ports on my laptop. I've already tried plugging the expression directly into the 12Step to reduce to 2 usb ports, but (from what I experimented with) that only controls volume in GarageBand; FL Studio reads the expression as a sustain pedal through the 12Step (only senses value 127, nothing else).
I think the main problem is the USB splitter (or maybe something to do with my laptop's RAM idk), but I have a feeling getting a different USB splitter will do nothing.
I don't think my 12Step could be buggy or anything, although I do find that I have to swivel the expression pedal by hand a few times to get the mechanism to work smoothly in FL Studio. As much as I'd like to just get a MIDI expander of some sort to see if that helps with this issue, I just bought an audio interface and am not trying to make another music investment rn. Let me know what you think.

[BandGeek32]

UPDATE 4/4/21 I think the problem may mainly be...

[BandGeek32]

UPDATE 4/7/21 So I decided to try ordering a be...

[kjetiln]

Did it continue to work for you? I have the sam...