Headphone splitter introduces minor latency

w10 Audacity v2.3.3
But the question is more generic than about Audacity (I think). I’m using a splitter so I can monitor live instruments and recorded tracks via headphones. When I split between live e-drums and Audacity output, the sound seems fine with no noticeable latency. When I connect a guitar alone, I hear it correctly. As soon as I also connect the pc output to the splitter, I hear both the live guitar and the Audacity rendering of the guitar - slightly out of sync. I have software playthrough unchecked in preferences, so I assume I shouldn’t be hearing anything except the recorded tracks from Audacity.
I plan on getting an audio interface, but will that resolve this?

I’m a little confused about your setup - A splitter normally sends one signal two places, such as into your headphones and into your computer, or into two sets of headphones. Are you trying to use it as a mixer? Two different sources into one headphone? That’s a bad idea because you are shorting two outputs together. (It’s OK to short two inputs together.)

I plan on getting an audio interface, but will that resolve this?

An interface doesn’t always make a good “live mixer” for monitoring. A Mixer with a USB port may be more appropriate. A USB mixer should be fine with two sources, but if the mixer has multiple channels the USB usually only contains the stereo mix. A multi-channel interface is used for multi-track recording but most of them won’t mix.

Reconnected everything again today and it seems to be working fine (of course!) Maybe restarting Audacity made the software playthrough setting stick?