Synchronizing different nodes on different machines

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What would be the best approach to synchronize two ROS2 nodes (i.e. a common shared clock/time) on two different machines, connected over a wireless network?

A non ROS solution: using ntp or chrony (or perhaps even something using ptp).

Time-synchronisation across different machines is expressly not a responsibility of ROS, as there are well established protocols and tools to do that, and it's largely orthogonal to what ROS was created for.

Is there any tool built into ROS that can help with precise syncrhonization?

No. See my earlier comment.

At present I've implemented a third node on one of the machines providing a clock,

Are you publishing to the clock topic? Note that topic is not intended to be used for (wall) clock synchronisation across multiple machines.

See also: Clock and Time on the ROS 2 Design site.

but apparently there is a slight delay/latency of 5-20ms on the other machine, which is variable, and thus hinders the accuracy of time readings.

Tools like ntp and chrony have built-in systems to compensate for transmission delays and similar challenges which come with time-synchronisation across multiple machines and using COTS networking infrastructure.