Hope you can find something that will live up to your 640's ability.
I agree with part of your assessment regards DRZ vs. DR650. If you use the DRZ as a road bike, certainly a 6th gear is needed. Would not be my 1st choice if fair amount of road riding is on the menu'.

I've toured A LOT with guys riding DRZ400-S, me on my DR650. In our group we'd have a few DR's and a few DRZ's. DRZ was just about as fast ... up to about 80 MPH. But the DRZ is working hard above 65 mph. The DR650 is loafing at that speed.

Off road the DRZ has big advantage, more fun and safer when things get technical or you want to push the pace. The "E" model, even better. I owned the E ... not street legal in USA so it was my Desert/Sierra/Mexico bike.

But on a Baja trip the DR650 was the better bike (for me) because of long highway sections. Also, RODE my DR650 from San Fran to Baja, then did 1500 miles off road, then rode the DR back home. A true dual sport ADV bike. You could do that on the DRZ-S but not quite the same experience on fast highway riding.

You can push the DRZ off road ... but pushing off road ... I'd prefer your
KTM 640. The geometry is just better, IMO. More confidence in front end. Tracks better through tricky ruts, doesn't get bounced off line. But all of them are too tall for me.

Sure, with lots of work the DRZ can be better but your 640 is already there.
Parts in short supply? Perhaps buy full set for rebuild, sit on them till you need them. That should extend things a few more years at least.

The hard part is having a bike that is good enough on the road yet still competent off road. On my DR650 I have to avoid technical stuff and slow down the pace.

On a 250cc I'm more relaxed and actually make fewer mistakes ... and I can safely UP the pace on the 250 where I have to be careful on my DR650.

On fire roads or easy two track, hard to beat the DR650. It does well, but still won't carve the corners with confidence like your 640 can.

Most really depends on destinations, length of trips and pace you want to travel at ... and of course what floats your boat. Big difference choosing a bike for bumbling round BC as opposed to riding out to Ulan Batar and back.

For short, local rides off road, hard to beat KTM 500exc. Leaving Canada?
Whole 'nother consideration.