The Fix:
Ensure the Clock Time for both computers matches each other in the same Time Zone. (Unsure if equivalent time in a different Time Zone affects it, as I did not test that.)
Reproduction Steps:
Every time I tried to connect with multiplayer, whether on the same local Wi-Fi connection or separate networks, regardless of which computer tried to host, the joining player received the message that they were unable to connect due to the connection being too unstable. This error occurred only while the clocks of the two computers showed different times, as one was a new computer that hadn't had its clock set to the correct settings.
Repro Rate:
Every time, while the clock times weren't synced.
In-Game Date & Time:
When trying to connect to multiplayer.
Players & Platform:
2, Steam, didn't matter which of us hosted.
Game Version:
v1.2b-1245
Input Mode:
Both Keyboard & Mouse and Controller
PC Specs:
Multiple, both on Windows 11
Epilogue: Since ensuring that the clocks for the computers matched, we have been able to play without issue and just finished our first Spring together. I figured I would post this here in case anyone else encountered this issue as I couldn't find this solution readily posted anywhere else that didn't take hours of research from both of us.