On my trip to Niagara Falls I took the 35 and 21 in a very small rubber lined pelican case. With the 18-135 on the camera you have a very light WP kit. Except of course you can't use the 35 and 21 in moisture. That's the way I'd go, the DA35 2.4 is cheap, the 21 not so much.
Images here. And here.
Another solution would be the DA* 16-50, but, then you probably want at least a 100 WR macro for a telephoto. To me the 18-135 is by far the best option for what you described. I haven't checked the DA* 16-50 against all focal lengths, but at 16 mm, compared to a 15 ltd, I found it wanting. To me, it's too expensive to not want on the camera for your best shots. I bought the 18-135 knowing it was coming off the camera for my best shots. Or to be precise, take the shot with your 18-135, then switch to an appropriate prime if you have time. That way, you can't lose. ANd as stated, the 18-135 has produced it's disappointments, but every now and then it knocks your socks off too.
Quote: In an ideal world I'd like a FF DSLR for when I need the "ultimate" performance & IQ,
Excluding the D800 or D3x, the K-5 is the "ultimate performance and IQ", and definitely the ultimate performance for $1000. At least for daylight landscape photography.