Thanks to all who have contributed so far.
Just to clarify, several things have me leaning toward a 5DS or 5DS-R (and away from a newer mirrorless)...
First, I have several APS-C cameras (a pair of 7DII are my primary users) and an APS-C mirrorless (M5). I also have an older full frame 5DII. And I have a BUNCH of lenses and accessories to fit them (except the M5). All but the M5 share the same batteries and chargers, too.
Second, I'm still using older post-processing software and don't care to upgrade yet (partly because it will mean building a whole new computer when I do). The s'ware I have supports the 5DS cameras, but not a lot of the newest models.
Third is cost. I missed the "mega deal" when Canon USA was closing out refurb'd 5DS models for just under $1100 (with full warranty) and the prices on used have rebounded. LOW MILEAGE (under 20,000 click) used are now selling for around $1150 for 5DR and $1300 or a little less for 5DS-R. Both models with more use and wear can be gotten for less. Sure, I know the advantages (and disadvantages) of mirrorless and an R5 would be great... but $3800? Several have noted that the 5DS-R is still available new (while supplies last?) for $1500, which is a possibility. I don't mind used with reasonably low mileage, though. With used I'd have no concerns about actually get out and using the camera, the way I might with new, at least initially. And I certainly won't be racking up the shutter count on a landscape camera, the way I do on my sports cameras! Rated for 200,000 clicks, a camera that's still got 80 or 90% of it's life left might outlive me!
There's also a significant jump in price with the RF lenses... While I appreciate the innovation and quality of RF glass, it ain't cheap and there are at least 3 or 4 I'd want to start, which add up to a LOT more than the camera alone! I I might add ONE EF lens for landscape work, but am pretty well outfitted for anything else I might want to do with the camera. The EF 16-35mm f/4L IS USM I'm most interested in can easily be found used in excellent condition for $750 to $775... or $1100 new. The most comparable RF 14-35mm f/4L IS USM is mostly only available new, out of stock most places anyway with no known arrival date, and costs $1700.
I don't know that I'd fit one to the 5DS/5DS-R, but use battery grips on most of my cameras, both for the added battery capacity and for the vertical grip and additional controls. Primarily planning to use the camera for landscapes, I might instead just fit it with an L-bracket (which I've also found avail. used at some discount). But I've found BG-E11 used for around $80, so might go ahead and get one to have, just in case. The 5DS/5DS-R are rated to give about triple the number of shots per battery charge that the R5 can do. I know folks often get better than the rated number of shots. I do too. Still, with an R5 I would at least need to carry additional batteries and might be more inclined to fit it with a battery grip, which sells for $350 (not avail. used) and sort of nullifies the mirrorless size and weight savings, as compared to a 5DS/5DS-R without a grip. The DSLRs, body only, weigh 30 oz. The R5 fitted with the BG-R10 grip weighs 38.5 oz.
Another added cost with an R5 would be memory cards. To take full advantage of the camera would mean buying at least some CFExpress cards, which ain't cheap ($150 to $200 ea. on average). I already have a number of SD and Compact Flash cards like the 5DS/5DS-R use. (Of course, an L-bracket adds some weight to a 5DS/5DS-R... or the BG-E11 would add about 10 oz. and make it a little heavier than the R5 + grip.)
As some have commented, and it's true, the 5DS/5DS-R have quite limited ISO range. Their native range is 100 to 6400, which can be expanded to 50 and just 12800 (I avoid expansion ISOs with most cameras). BUT, as a landscape camera it would be on a good, solid tripod a lot of the time (the same for any product photography or architectural shots), so somewhat longer exposures would be no problem. Besides, somehow I managed to take thousands of photograph for many years using 25, 50, 64, 100, 160 and 200 ISO color film, 32, 100 and 400 ISO B&W film, the last occasionally pushed to 800. Heck, usable 1600, 3200 and possibly even 6400 ISO would have been a dream come true back in the dark ages of film! (If you really want to discuss "relics", let me get some from my vintage film camera collection! Some are 100 years old and still usable.)
I may sell or trade-in of some of my older DSLRs to help defray the cost of a 5DS or 5DS-R, EF 16-35mm f/4L, BG-E11 and an L-bracket for the camera. Since I can get all four of those items for less than $2000, the money from the trade-ins (or sales) would go a whole lot further than with a new R5, etc.
Besides, Paul, I'm an old fart and really set in my ways (Scottish heritage). Don't particularly care to "change how I think about mirrorless, think about photography, or think about life"
Thanks again, everyone... You're feedback is very much appreciated. I rambled on a bit above... Do you think maybe I'm trying to talk myself into this purchase?
P.S. I do anticipate in the not-too-far-off future updating my 7D Mark II's to a couple mirrorless, APS-C format "R7"s, if and when Canon offers them. For fast action shooting, those will make sense and be a solid upgrade. Maybe a few years later I'll spring for the full frame, 80MP "R1" when Canon offers them refurbished or I win the lottery.