libexanic is a remarkably clean user-space kernel-bypass library that allows you to do processing on early fragments of the packet while the rest are still being received.
[0] https://www.shi.com/Products/ProductDetail.aspx?SHISystemID=...
But Netronomes are remarkably cheap, and you can "offload" eBPC to run on the card's custom cores.
https://www.cdw.com/product/exablaze-exanic-x10-network-adap...
Ultrascale. Significantly cheaper. Could it be made to work out is it subject to your comments concerning kintex, pcie straddling?
Aside from that did you know you were going to do this when you did verilog-ethernet etc?
I would argue that it strongly depends on the price differential and how resilient your system is. If your system is properly failure tolerant, and you can buy twice as much hardware for the same price by accepting a 20% failure rate (say), then it would be strongly advantageous to buy all used hardware.