Hi I'm 60, never had a mid life crisis! Been on bikes forever and ride in all weather 2 or 3 times a week and almost never on the motorway (no bends). Can't recommend any 1000cc modern performance bike to a new rider. When I started riding, they were two limitations

, money and the fact that most bikes were pretty low powered so they was an in built learning curve as money became available and power increased. There's a lot more to riding a bike than driving a car or simply passing the test. It isn't just a matter of throttle control as some have suggested. The ability to read the road, it's surface condition ,where it's going, the weather conditions and other road users all have much more bearing than when driving a car and have to be instinctively translated into road postion, balance ,throttle control and braking. This is what makes biking such a great experience. I think to enjoy a bike you need to be able to practise these things on a bike which has reasonable power before stepping up in engine size.
I see quite a few "born again" bikers who have bought high powered sports bikes and despite their previous experience, ride nervously, unable to enjoy their bikes because of the sensitivity of the controls and the speed at which the bike responds to input.
If you have to have a Multistrada, I would say go for the 620 but don't forget that there are other more friendly starter bikes out there. So what if you trade in after a year to get a 1000 cc? That's better than getting a bike you may not enjoy ( not because of the bike but because of your limitations) and being put off biking. Remember when roadtesters in magazines say the Multi is easy to ride, they are very experienced riders. It is an easy bike to ride in this engine size, if you have some experience. So get some experience on a smaller bike, then get a 1000 and enjoy it.
Good luck