Re: "Support for open firmware is, arguably, a niche consideration at the moment"
I was a big user and supporter of DD-WRT which is a user friendly take on OpenWRT. However TP-Link routers now contain most of the bits I was using in DD-WRT.
DD-WRT was a replacement for Linksys WRT54GS firmware but an arms race ensued where each hardware release from Linksys would seek to stop people installing DD-WRT.
There are loads of cheap routers that will accept DD-WRT but the last two versions on the TP-Link I use does not accept it. I don't think TP-Link have anything against DD-WRT just that they changed to a cheaper chipset. DD-WRT does support this chipset but someone has to create the version for this router.
Not such a big demand from me for this as the TP-Link does all I want like;
AP Router Mode
Range Extender Mode
which was my initial reason for using DD-WRT in the first place.
As for VPN you really need a more powerful router since these little ones can't do VPN at current high bandwidths.