In point-to-point protocols on a physical dedicated line, there is no point in such guarantees, confirmation is easy to obtain if necessary and it is not a one-way flow.
Here is the idea behind my suggestion:
[MCU]≣[W]=====[W]≣[MCU]From the MCU's point of view, the module
[W] is a reliable direct connection to the other MCU, just with some random latency (I'd say up to 10ms given OP's situation, using 10Base-T).
The purpose is to simplify everything, and isolate as much of the complexity to the separate module as possible.
Sure, you can use plain Ethernet frames. The module (or MAC, if you use an MCU with Ethernet support) inserts the preamble and start frame delimiter (8 bytes) and appends the interpacket gap (12 bytes). The header for 42 to 1500 bytes of payload adds 14 + 4 bytes, with many modules/MACs calculating and appending the 4-byte checksum. In wire terms, the overhead is 38×8 bit times per payload chunk, so it varies from 47.5% for 42-byte payload to 2.47% for 1500 bytes of payload. For 2Mbps data payload stream, it's in all cases less than half the theoretical bandwidth of 10Base-T, so should work fine. You would need to consider how to handle e.g. occasional data garbling (due to transient EM-coupled noise, perhaps) yourself.
Given a suitable Wiznet module, the MCUs only need to configure the modules, and then they can treat the 10Base-T cable run as a reliable transport between the modules, and only worry about talking to the module (which is simple). They ensure that data in the TX buffer of one module will end up copied to the RX buffer of the other, handling very short interruptions and noise via retransmission automatically; anything major, and the MCU will see an error. So, while not in any way optimal solution, it is a low-cost and simple one.
Personally, I'd use my Teensy 4.1's with Ethernet instead, and most likely raw Ethernet frames. I don't know SuperFungus' preferences as to how deep into the software they want and are able to go, but I do believe the Wiznet option is much easier to implement than the Teensy one is. Simply put, I suggested something I thought might be useful to consider, even though I myself would choose a different path.