ETTR is the reverse of exposing for the shadow.
Exposing for the shadow you watch your shadow make sure it has enough exposure and don't care for the highlight. You fix it in the darkroom.
ETTR you watch the highlight make sure it's not overexposed and don't care for the shadow which you can bring it up in post.

Preset is not fixed aperture. It's 3.5 to 22.

ETTR could mean increase or decrease exposure as per the meter depending each scene. A flat scene generally mean increase exposure. A high contrast scene could mean decrease exposure.
ETTR only mean that you expose the important highlight near the washed out point but not yet washed out.

Fixed aperture lens doesn't need any blade.

ETTR is not overexposure. Rather watch for overexposure. Give as much exposure as possible without overexposure. Watch the highlight and make sure it doesn't wash out.
Back in the negative film day you could exposure to the left that is make sure you have sufficient exposure for the shadow.

Best way to find out how much it was worth brand new. You will find it wasn't worth much when new.

I use a bunch of old Nikon lenses on my Df.
20mm f/2.8 AF-D
28mm f/2.8 AF-D
50mm f/1.4 AI (no AF)
85mm f/2.0 AI (no AF)
100mm f/2.8 series E (no AF)
135mm f/2.8 AI (no AF)
24-85 f/3.5-4.5 AF-S G (non VR)
70-300 f/4.0-5.6 AF-D ED (non VR)

The SLT was done solely for phase detection AF.

So to speak it has some source of mirror. Doesn't have an optical viewfinder. It lose some light due to the SLT spliting light between the viewfinder and sensor.

I don't think Nikon ever made the SB810.

If you like it buy it. The price is not going to be any lower ever for a brand new camera.
I personally don't like it. I in fact like the Sony better. I am talking mainly about look as the performance are about the same.

Since you don't really have that many lenses for the FM2 you can start from scratch. I suggest that you check out the mirrorless Olympus and Fuji. Pay attention to the viewing system and compare that to the DSLR. Viewing including manual focusing. So compare the 2 systems and see which you like best then go from there. I stay with a DSLR because I like to manual focus and compose on an SLR viewfinder and not the EVF or LCD. That is the determining factor for me.

I used film since 1977 and didn't go digital until X'mas 2013. When I moved to digital all I bought is a DSLR body and that's all. Everything else are the same as in the film days.

To have the same view (field of view) the larger format must have lens of longer focal length.
The larger format should also has larger circle of confusion because there is less enlarging to make same size print.

For example to use a 4/3 camera and a 25mm lens would give approximately a full frame 35mm with a 50mm lens. If the image taken by the 4/3 camera at f/8 then the image taken by the full frame camera but be f/16 to to have the same depth of field.

If you look at the DOF formula you will see.
1. Hyperfocal distance

H= f^2/N*c H=Hyperfocal distance, f=focal length, N=aperture, c=circle of confusion.

2.The near and far distance with acceptable focus are:

Near distance= H* s/H+s where s=focused distance
Far distance=H*s/H-s
3. As you can see if the image has the same DOF they should have the same H.
4. Look back at the formula for the hyperfocal distance. For a format that is twice the linear size the focal length has to be double and the circle of confusion is also double. Since the focal length is squared it make the numerator 4 times greater but the denominator is only twice greater. To make it equal the N (aperture has to be doubled also).
5. So for a format that has twice the linear dimension the aperture value has to be double to have the same DOF. Double the aperture value mean stopping down 2 stops.