1) Use the cameras built in Long Exposure Noise Reduction. This will double the time for each exposure.
2) From Photographer Jon Secord: Here's a quick and easy way to remove those hot pixels in photoshop if you don't have a dark frame to work with- -While doing your raw conversion in LR crank up the Color Noise slider to 100%. This will desaturate all the hot pixels but not remove them -Once in photoshop duplicate your background layer. Zoom in to 100% and then open the dust & scratches filter on your duplicate layer. I keep the radius as low as possible (usually between 1-3) and then play around with the threshold slider. You want to remove all the now white hot pixels but not go any further with the sliders. This is going to blur your image a bit- that's ok. -After applying the dust & scratches filter, change the duplicate layer's blending mode to darken. This will bring back all the detail in your image but essentially remove the hot pixels. I've been using this method for the last year or so and haven't noticed any reduction in the foreground detail, but it completely removes all the hot pixels. Once you dial in the process it takes like 2 minutes. Since you're already combining a longer foreground exposure with your sky frame, make sure you do this to your FG layer before blending in the sky. The dust & scratches filter will also remove a bunch of your stars (since many of them will be the same size as the hot pixels). I definitely get a bunch of hot pixels doing 4-5 minute foreground exposures with the 750, but my 6D was so much worse. I remember viewing my old 6D raw files at 100% and it looked like confetti cake....hundreds of colorful dots.
3) From Photographer Matthew Saville Baldon: I'd strongly recommend checking out Capture One Pro 9 for long exposure editing. It has a "single pixel noise" slider that is *magic* for exposures even 30 minutes long! I'm not a fan of the mushy digital-y noise reduction and sharpening recipes that Adobe offers, however for those who are then admittedly Capture One Pro 9 is slightly less powerful when it comes to overall luminance noise reduction compared to Lightroom / ACR. Like I said though, I usually leave a natural looking of luminance noise in my nightscape shots, so I haven't felt held back by Capture One's other NR or sharpening parameters. That, plus the vastly superior color and tonal processing ability versus ACR, has won me over for much of my landscape work. If you can shoot your shorter exposures first, and then afford the time to let your camera "idle" after your >60 sec exposures, I have found that Nikon's built-in long exposure NR can work wonders. The only drawback is that if you do a 30 minute exposure, most Nikons need a whole 'nother 30 minutes to do the LE-NR, which is a show-stopper unless you have two cameras. Honestly though, I've had GREAT results from Capture One Pro 9's "single pixel" noise reduction option. It works perfectly every single time, on literally every single dot, unless you start actually getting seriously dead pixels clumped together. And it doesn't really harm image detail any more than your long exposure already is going to, either. (For what it's worth: The D750 sensor, thankfully, does *very* clean long exposures when you get below freezing, by the way)
4) From Photographer Ryan Moyer: You can always just shoot a dark frame for the hot pixels and amp glow. After your exposure just put the lens cap on and shoot another shot with the same exposure settings. Then take it into photoshop, open the two exposures up as layers and change the blend mode of the dark frame to 'subtract'. That will eliminate all of the hot pixels and amp glow. This is basically the same thing that long exposure noise reduction does, the only difference being with the manual process you can just shoot one dark frame every once in a while (namely when the temp or your exposure settings change) rather than after every single frame like LENR does. Hot pixels and amp glow are going to crop up with long exposures at high iso for any camera if it's warm out. I'm surprised the 6d is even better than the 810a as the 810a is already one of the best in that regard. I have to manage the hot pixels on my A7r even when it's 40F out. On my D750 I only have to worry about it on those warm southern Utah summer nights, similar to your 810a.