Seems as though it's going to be difficult to plan far ahead with the development of this winter's snowpack still to unfold over the next 4 months. Late Spring means different calendar dates at low elevation than at high elevation, where we might consider late April to late May as Springtime down low but must consider late May to late June as Late Spring higher up.
I'd be checking with the authorities at Rocky Mountain NP as to when Trail Ridge Rd opens up. The NPS website mentions earliest ever opening as May 7 with the latest ever opening being June 26. If you're routing to Estes Park from Denver in order to traverse Trail Ridge Rd headed west/northwest, it may not be possible.
In general, I'd be looking at US 287 out of Ft Collins up towards I-80 at Laramie as a scenic drive through lots of NF and BLM lands as well as one which holds fairly high elevation throughout, but which is a major shortcut route avoiding the 8,640' pass between Cheyenne and Laramie along I-80 so as a result is well traveled and plowed. From Laramie to Rawlins, I-80 runs at between 6,500' to over 7,500' with abundant access to even higher elevation public lands. There again, US 287 north of Rawlins is high elevation intermountain cruising with fairly easy access to public lands on the north side of the Wind River Range. US 287 proceeds through high mountains all the way from Dubois to Moran Junction north of Jackson, WY at the entrance to the Grand Tetons NP.
Also in general, Late Spring at all elevations translates to higher water levels (runoff season) on streams within and exiting mountain ranges. Since practically all routes accessing higher ground begin by heading up a canyon along a stream, you may find high water issues at bridges and at campgrounds as you enter higher terrain.
From Moran Junction you may continue into Yellowstone if the roads exiting to West Yellowstone, MT are open or run south through Jackson to cross Teton Pass to enter Idaho, head west/southwest to Twin Falls, and take US 93 down to Wells, NV to pick up I-80. Most of that is along the Snake River plain so is low elevation, but you're never too far from some high ground for overnights until you reach Wells.
Edited by Foy, 17 January 2019 - 10:21 AM.