Gaining

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.

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.

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.

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.

Have no clue what mine are as I've never been tested lol...good luck, let us know how fat ya get

What kind of life are you expecting or preparing to have at 30? Do you have family/friends/enablers/helpers who are willing and not see you as a burden as you become dependent?

You'll need to make more lifestyle changes until your metabolism slows down with age. How is your relationship with food? If eating feels like work, you'll be less likely to do so, same with food you do not have an emotional connection to.
More like minded people, will help as you adopt their habits, mannerisms over time.

FatLover91:
I have family who would be able to help me once I’m immobile and no longer able to care for myself. My relationship with food is great. I love to eat and it doesn’t feel like work to me at all. I am looking for a feeder though too.

As long as they do not see you as a burden but a labor of love, as it is not easy.

Has the family been told of any of this or are you assuming they will be OK with it?

What if they ask you to move out?
What if they kick you out when they figure out what you are doing?
Its their house they have whatever choice they want.

They'll have the choice to kick you out long before you get that big or get a court order to send you to a mandatory weight loss clinic. If you're medically unable to take care of yourself, your immediate family has legal right to do what they want. You think for one second they'll be okay with you immobile?