Would this router be supported (http://en.techinfodepot.shoutwiki.com/wiki/TP-LINK_Archer_C5400X_v1.x)?
What does https://openwrt.org/toh/start say?
BCM4366E WLAN is fullmac based and seems to be supported by brmcfmac, but Broadcom (BCM4908) ARM and NAND flash (F59L1G81MA-25T) are generally challenging, due to the way Broadcom partitions their bootloader on these SOCs. While I wouldn't rule out potential future support completely, the situation is unknown at best, pending someone with the device on their desk to spend some quality developer time on it - I wouldn't place that device on the top of my wishlist.
I just wanted to ask, since, I have this router sitting on my desk with its really tuned down stock firmware, which I cannot do anything with. Is there any way for me to build OpenWRT myself or something (though I really do not want to open it up)?
No, unless you take the long route, by developing the necessary support (https://openwrt.org/docs/guide-developer/adding_new_device https://openwrt.org/docs/guide-developer/add.new.device).
Ok, Thanks for your time. I just have a load of routers (mostly chinese) that don't support any custom firmware, which I am trying to do something with.