It's been awhile for me so maybe others can give you more detailed and up-to-date information. I remember taking that exact route you've planned on taking and it is very scenic. However, if you're worried about grades --and grades alone-- you might want to take a different route.
U.S.191 hits almost 10,000 feet in elevation twice. Once at the Indian Summit (elevation ~9,100 ft.) and then again going over the Uintah Mountains where there are lots of switchbacks. The roadway itself is good from what I can remember but, as I say, if you're worried specifically about grades, I'd probably recommend taking an alternate route. Like I say, maybe others will have different opinions.
As far as alternate routes, I guess you could go over Douglas Pass on U.S. 139 as that's not quite as high in elevation but still could be taxing as I seem to remember it having 7% grades in short stretches but the summit itself is under 9,000 ft. It would be this route shown on Google Maps
HERE
The only other route I can think of that may take you more out of the way but might be more level is to remain on I-70 to CO-SH13 to CO-SH64 to U.S. 40. I've never been on this routing so I can't comment on it first-hand but maybe some others can. The grades are mostly on the interstate so you can just take it easy and just keep pace with the semis. It is shown
HERE on Google Maps.
Lots of forum participants from Colorado on the forum so I'm sure they can give better feedback.
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