I'm trying to use a low power wifi connection to move about 20MB/sec of data between a instrument mounted on a rotating steel barrel about 20 inches in diameter and a stationary wifi unit. A low power solution would be nice but not critical. I'm looking to either buy a TP-Link - Archer AX20 AX1800 Dual-Band Wi-Fi 6 Router (https://www.bestbuy.com/site/tp-link-archer-ax20-ax1800-dual-band-wi-fi-6-router-black/6442841.p?skuId=6442841) on the stationary side with the 4 antenna spaced equally arounds the drum (the equipment is enclosed in a steel box) and a ESP32 or similar on the rotating part. It occurred to me that it may be better to have one receiver input and a 4 way splitter. Any advice would be appreciated.
You definitely don't want to feed all 4 antennas to a single receiver input, since WiFi 6 is MIMO - Multiple Input Multiple Output. Each antenna actually carries a different signal so that the over all transmission is multiplexed in both code and space.
You are probably fine leaving the router just the way it is. At 5 GHz the signal is almost completely spread by multipath so that there is already little correlation from one antenna port to another. If you were to disconnect the antennas from the router and cable them to some other location, most likely all that you would be doing would be adding attenuation loss to the antenna-router path and lowering your available bit rate even further.