You are looking at the current carbon-framed Sirrus, yes? 35s should be no problem; 38s you'd want to check on, but for the kind of riding you describe 35s or even 32s should be more than adequate.

Choose your bar type (flat or drop), choose your desired tire clearance, then choose your bike. Lots of options these days, with the proliferation of so-called 'gravel' or 'adventure' drop-bar bikes -- which are, really, just drop-bar bikes with sensible tire clearance. Ignore the marketing nonsense.

As to riding something like a Sirrus on non-technical off-road? No problem with sensible tires. I ride mine (2010 w/32s, which is all it will swallow; pic below) on gravel and dirt all the time. The trick is to 'ride light', using your arms and legs as shocks and keeping your rear end off the saddle when necessary. I'm 65 with lots of physical issues, and I've no problem.