So lots of good suggestions here... but I'm still left with several options with pros and cons for each... I figured a picture is worth a thousand words, so here are diagrams for the 4 possible solutions:
Below is the existing situation (green is new 1.5" Saniflex to replace my existing 1.5"... already done... blue is 2" hose we're discussing how best to replace:
Below is Proposal #1:
This would replace the 2" hose with 1 7/8" hose... This is the simplest route *IF* I can squeeze that small hose onto the (slightly too big) fittings... The drawbacks *IF* it fits, are that it costs slightly more (no big deal) and that I'm using slightly inferior hose (certainly less-smell-proof than PVC) compared with the Saniflex hose. I'm not looking forward to getting the hose on the fittings if this is the route I go.
Below is Proposal #2:
This would use the best possible hose for the job, which I happen to already have on hand, but requires swapping, or reducing tank fittings (not a big deal) and swapping out the deck fitting... which requires me to make the bigger hole in the deck into a smaller one, and purchase and use a non-matching deck fill, I hate to get into filling holes in the deck, redrilling, etc. etc. more than absolutely necessary. The hose is easiest to run this way, and saniflex is great (though maybe not quite as good as PVC in this use)
Below is proposal #3:
This would be very smell-proof, but potentially tricky to fit PVC in place (and would require cutting of PVC if I ever want to remove it (not a big deal, PVC is cheap and easy to work with) This also uses all-new-saniflex 1.5" hose for a small section but requires a new deck fitting as well (see disadvantages already listed for replacing deck fitting).
Below is proposal #4:
This takes advantage of the smell-proof PVC... and smell-proof was the entire reason I'm doing this project in the first place... and costs the least (as it only requires purchasing of maybe 1 foot of new 1 7/8" hose). It also has no smaller 1.5" bottlenecks in the system... but also relies on fitting 1 7/8" hose on a 2" barb... which may be a challenge. Presumably in any of these solutions that contain part-pvc-part-hose, I would do the maximum possible PVC run, and the smallest possible section of hose... really the hose is only there to get from the deck fitting barbs to a coupling transitioning to PVC below as quickly as possible. I suppose a slight variation on this one (which I'm not going to redraw) is to get just 1 foot of 2" Great White hose, shipped for like $50... ridiculous, but I suppose not impossible. Also possible is to use 1 foot of something non-sanitation-grade... I don't love that idea, but it is way easier to get some other 2" hose... and it could be kept to an extremely short length... really just inches long to put on the barb of the deck fitting, and then immediately clamped to another coupling onto PVC.