>>28683233
You can take a lot of bikes down dirt roads of varying shittiness with the skillset and balls. Small wheels really want to follow any disturbances in the road, or just not roll over bumps very well. These qualities are less than ideal for dirt. The 19" is less bad about this and has knobby tire options. The BMW GSA uses a 19" and that's the poster child or ADV. The 21" is the standard the industry has decided on for many years now. If you want the luggage, you're just going to have to spend extra on aftermarket boxes.

Another option for you might be a used Africa Twin. Big, comfy, powerful but not insane, reliable engine, 21" front wheel as standard prior to the latest generation.
>>28683235
36 is very young unless you take shit care of yourself + work a blue collar job. I would buy the Z900RS just because it's gorgeous and reasonably comfortable unless you see yourself doing a lot of aggressive riding and/or track.