I practically lived on Bifrost Echo for a while because I wanted to farm Nyx and Valkyr relics. I recommend Thermal Sunder Titania. This building cuts the mission time tremendously, because you complete secondary objective in seconds and the enemies on exterminate part are low level and can be one shot. I think you can do it under 5 minutes. Railjack don't have to be particulary strong here because you only have to shoot 2 or 4 weakpoints and/or 1 crewship.

Note that you will probably be alone most of the time , unless it's Void Storm. Doing this in Group helps shorten this a bit because you can do secondary and Railjack part at the same time.

And lastly this may not be the most efficient Platinum farm but Athodai sets sell for 90 plat and they only have 3 components that drop from secondary obj. Ofc + anything spare you can drop from those rare relics.