Here are my thoughts and essentially how I did it/do it. First off, it's more difficult to trade with cash on top. It's possible, but you won't likely be able to do this with each transaction.

Instead, sell your machine for market price and get the cash. When you go to purchase your next machine, add a little bit more, whatever you're comfortable with and can afford. This might mean buying machines you don't particularly care for, but it means you're building your pinball pot a little at a time. Obviously, if you have more games in your collection, this will be a more flexible transaction as your pinball pot is growing as you upgrade, but you won't necessarily have downtime without games.

Buy projects, fix 'em, sell 'em. These games exist, but you'll have to go looking for them. FB, CL, Pinside, they're around. Don't discount EMs. You never know, you might even enjoy them. If you aren't comfortable working on games, get comfortable. I've learned most things I know from Pinside. It is an invaluable resource. Despite the regular jokesters on here, there are just as many that are way super smart and know these games in and out and are willing to help.

Buy games outright, slowly building your collection, adding to your pinball pot one game at a time. If you have the extra cash, get another machine alongside your Spring Break. Then you can sell your Spring Break, and add a little extra on top for the next game, while still having a game to play at home.

Times are a bit different now, but this is essentially how I built my collection and can afford more expensive games. I don't make a bazillion bucks a year. I can't buy every NIB game that comes out. I pretty much can buy what I want, but have to sell to make funds and space. In time, you end up with a collection that stays, with may be 1 or 2 rotating spots that are up for sale if/when a more desired game of your choice comes around. When it becomes hard to sell a game in order to get another one, then you're in a pretty good spot imo.