Hello @TempleofTyler
Getting a good connection to an outbuilding is best done using an Ethernet cable. But, it has to connect to the primary unit's LAN Ethernet port, not to either of a secondary unit's Ethernet ports (e.g., your kitchen unit). Here's a support article with more details: https://support.google.com/googlenest/answer/7215624?hl=en
Attempting to build a multi-hop wireless topology (e.g., hoping your garage unit connects to your kitchen unit) does not typically work out well. In addition, building materials can often cause significant signal degradation. Windows often have coatings that impede the already very low power WiFi transmissions, for example. Aluminum siding, metallic vapor barriers, brick, plaster, etc. all cause problems.
Lastly, Nest WiFi Pro uses the 6GHz radios to carry the wireless mesh interconnection traffic (in addition to 6GHz client traffic). Currently, this is limited to a lower transmit power, which reduces the range. So, the optimal placement for a wireless-only system has the primary in the center (which it sounds like you already have) and secondaries one (or, at most two) rooms away from the primary. From there, they will provide 2.4GHz, 5GHz, and 6GHz coverage to more distant clients. Wiring a secondary directly to the primary (or through an inexpensive Ethernet switch connected to the primary) skips this, allowing you to place a secondary further away, or in an outbuilding.
