Nikon already have (one of) the smallest lightest mirrorless cameras available.

If you don't mind the 2.7x crop factor, compared to what you are currently used too, it could do an ok job as a travel camera.

About $500-ish gets you a J4 + 10-30mm kit lens, which is an OK start point.(Aussie retailer prices .. possibly cheaper from grey retailers??)

For more lens options, you could just use your current lenses(AF-S only) with most of the automated features available, (ie. AF, exposure, etc) via the use of an FT-1 adapter.

The only issue you have with the smaller sensor formats is the availability of wide angle lenses for them.

That's option 1:

Another option, and only because you have listed size, weight and now price as important considerations .. instead of the 28/1.8G .. have a look at the old 30mm f/2 AF-D lens as well.

It'll be much cheaper in price, by a considerable margin .. I vaguely remember them at the $300 or less price point.
It is old tech, in that it uses the dreadful AF-D focus system, but if you don't mind that, it can be a pretty useful once off purchase for your purpose(due to the low initial price).
Then if you find that you don't use it much, if you sell it and lose $50 or so on it .. you won't have lost all that much money.

You won't notice much subject separation difference between f/1.8 and f/2 at 30mm!!) Of course the numbers will show that the faster aperture lens will have more isolation/blur, but in human vision terms I doubt anyone would notice it.
It does give you more subject separation at 35mm and f/2 for the same subject distances as a 28mm f/1.8 would allow.
I suspect tho that with the same subject size in the frame(ie. getting closer with the 28mm lens!!), it would again be a hard task to differentiate the isolation levels.


Size and weights:

Lens ........ diameter/length(mm) ....... weight(g)
35/2 .................. 64/43 ......................... 200
35/1.8 ............... 72/71 ......................... 300
28/1.8 ............... 73/80 ......................... 330

I have a recollection of using the old 35/2, and I remember it was one of the better focusing AF-D lenses .. still too loud, but not like my old 80-200/2.8. Fast enough, due to the overall smallness of the lens. IQ I remember wasn't too bad. Not stellar, but you probably wouldn't be unhappy with it's ability.

The key points to the 35/2 are probably right up on your list of priorities .. very small and light weight, and cheap enough that it justifies it's purchase.

ps. just had a quick squizz at some online prices for this lens(Au stores) and it comes in at the low $400 range. Too much. I remember this lens used to be in the low $300 range years back .. just after I got my D800E(mid 2012). This is an old lens, and should never increase in price.
I'd check ebay if you want better prices.

hope that helps.