Sigma lenses: using Metabones Speedbooster vs DSLR?

So there are two Sigma lenses that I am considering purchasing:

Sigma 18-35mm f1.8
Sigma 120-300mm f2.8

The 18-35mm being an APS-C lens will effectively be a 12-23mm f1.3 lens on MFT. I'll be using this indoors for stuff like pet portraits, so I am not too concerned about weight. 12mm is one of my favorite focal lengths, and the possibility of having both the Panasonic 12mm f1.4 and Olympus 17mm f1.2 Pro in a single lens (more or less) is intriguing.

The Sigma 120-300mm will be used as a 84-210mm f2 lens for mammal portraits on in safari vehicles, where weight and handling won't be a huge concern. I know that it's an absolute dog in terms of weight and it certainly won't replace my 50-200mm SWD or Rokinon 135mm f2, which I love. I do not care about AF speed because land mammals aren't all that challenging to shoot.

My question are is whether it would be better to use them with a Metabones ULTRA Speed Booster, or with a high MP DSLR.

Regarding DSLRs, my choice would be between the Canon 5DS(R) or the Nikon D850.

Advantages of Speed Booster:

Improved sharpness over the bare native lens
No need to buy a new body and all the headache that goes with it (batteries etc.)
Greater pixel density of M43 bodies
One stop improvement in low-light performance
Ability to use an EVF for focus fine-tuning etc.

Advantages of Canon 5DS(R):

50 MP sensor
Much cheaper used than the D850
Able to use EF-mount lenses with AF on MFT bodies if needed
AF advantages of shooting on the native mount

Advantages of Nikon D850:

45 MP sensor
Better AF and dynamic range than the Canon 5DS(R)
My familiarity with Nikon bodies
Improved burst rate if using a battery grip
AF advantages of shooting on the native mount

Anything I might have missed?

I'd love some feedback on whether it would make more sense to use these lenses with the MB ULTRA SpeedBooster or on a native DSLR, and if the D850 or 5DS / 5DSR would be a better choice.

For cost and other reasons, I'm not interested in the Sony A7Rx / Nikon Z7 etc.

FWIW: The Sigma 18-35/1.8 with Metabones Ultra Speedbooster shows up as a 13-25/1.3 on my OM1 and GH6 displays. However, I did not check for actual FOV.
12mm on MFT would be equivalent to 16mm on a crop 1.5 APS-C format (15mm for Canon's crop 1.6).

The Panasonic Leica 10-25/1.7 has been available for nearly four years now, it made the adapted 18-35 obsolete for me. Also, I wonder why you don't consider the Olympus 40-150/2.8 Pro. Native lenses offer better AF, and additional system specific goodies.

My 2¢: better rent the various setups you are considering and test them yourself before you buy anything.

I already have the 50-200mm SWD which I am pretty happy with. I do not need the best AF for this range so I use the SWD. I actually owned the Panasonic 50-200mm for a while and sold it because I mostly need this range on safari (occasionally) and could not justify the cost of the lens. My primary lens is the 300mm. And I love the kind of look I've been get from my FD 300mm f2.8L (with / without focal reducer) so I think that the Sigma 120-300mm would be even more useful, particularly if I use it with a Speed Booster, or with a large MP DSLR (not looking at FF mirrorless because of the cost).

Renting is good advice, and it makes sense to wait and see what OMDS has up their sleeve for the upcoming zoom lenses. :) I hope that one of them comes with a built-in TC.