After spending a few more hours going through archives, I’m no closer to choosing long glass for the SL2-S.
All the possible repeats have downsides I’d like to avoid:
Nikon 300mm PF + TC: Plenty sharp and lightweight, great under clouds and overcast skies; but has a propensity to lose contrast not only against backlight but with harsh front or side light, exacerbated by the 2X. The 500mm PF, which I’ve never owned, supposedly suffers the same affliction. Slight sample variation is another possible issue.
Sigma 100-400mm: Decline in acuity at the long end, unreliable OIS, questionable focus accuracy possibly in part due to DFD AF.
Leica 90-280mm SL: Superb lens, but far too short for its weight, and for my purposes.
Leica 280mm f/4 APO-Telyt-R + 2X: Best of the lot in terms of archived performance. But old, no longer serviced by its maker, some parts no longer available, and with weight that goes against both its own longevity and that of its user.
Sigma 150-600mm: Very good optics, dependable stabilization, but with the DFD issue. Also deployed weight (hood, tripod foot, filter) is around 2.5kg. If I were still willing to lug a 3.5kg setup around, I’d probably risk another copy of the 280mm f/4.
Canon 400mm DO II + 1.4X + MC-21: Never owned/used, but another heavy setup, also IMO overpriced even in the secondhand market. Analog MF would offset the DFD issue.
Lumix S Pro (any): No. Just no.
Priority is one that’s not a backbreaker, as I often walk two or more hours without sitting. There are a couple of adaptable options out there that might work but for focus-by-wire MF.
That’s about all I’ve come up with so far. Suggestions via email are most welcome!