A DLNA server would require resources that a Roku player just doesn't have. It would require more memory and more processing power. I can't speak to the USB power, as I've never used the USB port for anything. I stream from a DLNA server on my home network, or use Amazon, YouTube or Netflix.
It sounds more like you just want to be able to view your Roku on a remote display. A DLNA server wouldn't be able to do that. You would need some way to mirror the Roku to another device. I'm unaware of any way to do that.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.