Basically, for anyone trying to make sense of any of this:
AX5300 is a speed rating based on Wi-Fi 6 standard; if two different devices follow the same standard and have the same antenna setup (to oversimplify), then the AX number is exactly the same, even if devices have different ports and different chipset/CPU/RAM/ROM/GigE/USB/software/drivers, even if such changes mean that real-life performance would be different;
MX55EC3 is presumed to be a BestBuy SKU for 3x MX5500;
MX55EC3-RM2 is Linksys Certified Refurbished;
Looks like both MX5300 and MX5500 may be supported by OpenWrt, but clearly MX5500 is a huge downgrade.
Personally, it may be a fair price, but if you want OpenWrt, it’s not the best deal by any measure:
Lack of USB is a huge blow.
I think there was some talk that 512MB RAM may also be too low for the required wireless driver from OpenWrt for some of these QualcommAX devices to work properly.