From limited market research I've done your wife's photos and vids would probably sell the best if only one of you was going to do it.
However male gaining vids are kinda rare so even starting small you'll probably develop a fan base. I wouldn't worry about your starting size, people enjoy watching models grow.
I strongly encourage you all to explore mutual gaining content. It's got almost unicorn levels of rarity and is a fairly untapped market. Most content marketed as mutual gaining is set up as model x viewer and not 2 models gaining together. What is out there for couples leans heavily towards lesbian couples gaining together.

If you want to be 300 pounds then go for it. It's your body, make it look how you want to look.

You want to be fat. At some point you're going to realize that whatever you do with your life somebody is going to judge you for it. When you reach that point you can either fold to the pressure or say fuck 'em and do what makes you happy.
I won't say it'll be easy, there are a lot of fools who think they know what's best for you. You'll just have to learn that their opinions don't matter. Living your best life tends to tick people off, but I'm sure you already know about that.

To be fair though as an American your cholesterol and blood pressure probably aren't great to begin with. Something like 85% of people will end up on meds for at least one of those.

Chances are most the people you know over 35 have meds for one of them. I'd say 30, but I'm giving everyone a few years to get things diagnosed. At 33 the only person I know not on medication for one of them is a triathlete.

You're looking to gain 275 pounds in 3.5 years. Chances are your diet will be less than stellar.
Medical issues are fairly likely. If you were looking at a longer run up it would be easier to keep everything in balance. Refined sugar makes it really easy to get higher calorie density. Unfortunately going overboard on it isn't great for your body.

Between the 2 I'd choose belly rolls. I prefer hanging bellies to round ones. My personal preference is apron belly as one big slab of belly.

Shakes or cream can both help boost your calorie intake. Keep in mind you don't want to do cream long term, it'll wreck your cholesterol.
I don't have a particular recommendation on shakes. From my experience nobody agrees on the textures and flavors. Just trial and error to find what works for you.
Also try not to go too overboard on sugar and salt. Chances are you'll end up on meds for high cholesterol, high blood pressure, or diabetes at some point since you're going for 600. However you want to put it off as long as possible.

So is it possible? Yes, you're looking at about a pound and a half a week. We've seen several people over the years keep up that kind of gain for a year or more.

Is it likely that you'll maintain that gain? Probably not, or at least not solo. If you have a feeder to help you it's much more likely. Most people hit a wall between 400 and 500 where they can't get fatter solo because of the sheer calories needed to gain gets very high and is only higher if you have to move to get every bit of your own food.
If you want it, then by all means go for it though. Even if you're not 600 at 30 you're bound to be quite large with even moderate success.

Honestly I'm a bit weird here. I'm an Aromantic Asexual, so I'm not the average FA. However I've also always found fat very aestheticly pleasing. To that end I knew since I was little I wanted to be very fat. My parents were fat, their friends were even fatter, most my friends parents were fat. How could I not like it? Admittedly I wanted to be fatter then them (and I've almost reached that point) so my initial goal was very big.
Additionally confidence in your look is key to looking good. Someone who has the body they decided to have absolutely own that. Doesn't matter if you decided to be a body builder, be built like a bean bag chair, or just own the dadbod, if it's how you want to look then you'll look good.
So I decided I wanted to be fat, back when 200 sounded huge. I wasn't looking for any kind of relationship, so I labeled myself a gainer and haven't really looked back. My goal numbers have shifted over the years, but I'm still striving for my big apron belly.

It really depends for me. I'm 385 at 5'10" and if I'm just standing around my lower back will start to hurt. If I'm moving around it isn't near as bad.
I suspect I'm leaning back to counter balance the belly and like with pregnant ladies it's pulling everything out of place.