70-200 (and esp f4 version) will somewhat limit you to outdoors and relatively bright scenes. I’ve travelled a lot, both for work and for pleasure. In the beginning I would take a ton of lenses for “options”. If you want one lens for travel versatility: 24-70mm f/2.8.
But if and when you get into prime lenses: 35mm f/1.4 is my lens of choice for travel. Those are the shots that I consistently return to and truly love. For example: inside food stalls of Singapore, inside cafes in Italy, etc. these are all relatively low light as well …so the ability to opening up your aperture will give you nice creamy backgrounds but also the ability to use relatively normal ISOs.
It all depends if you’re set up as a business or not. Your last sentence regarding shooting weddings is kind of vague.
You would need to talk to an accountant to get any real advice, but I would imagine they would first ask how you’re currently conducting your photography.
Whether you sell your MLS pictures or not, if you were a legitimate business you could absolutely deduct certain expenses related to that. Working photographers shoot personal work all the time and if it’s in support of their business then there is a path for a write-off (in some capacity.)
Pro-tip: just because you aren’t selling these MLS pictures now, doesn’t mean you can’t sell them later.