Many side gigs can qualify as a business for credit cards - tutoring, babysitting, rentals, selling on eBay, etc. You would apply as a sole proprietor and use your name as the business name. This doesn't help you with Chase business cards though, as they don't seem to be approving sole proprietors much these days. You are 3/24 - 3 new personal cards in the last 24 months. Chase won't approve you once you get to 5/24. Business cards don't go towards this count as long as they don't show up on your personal credit report (most don't). If you choose not to go for business cards, then I think your next cards should be Chase cards. If wouldn't make sense to go for personal cards from other issuers, take up the last 2 slots, and lock yourself out of Chase. In this case, I would consider the Hyatt and the Bonvoy Boundless (which is currently elevated at 100k).
The Hyatt is a nice keeper card if you can use the category 1-4 free night every renewal (after the first year) on something that costs more than the $95 annual fee.
Both Chase and Amex have Bonvoy cards, there are some complicated rules about which cards you can get when you've had others. If you get the Chase Bonvoy Boundless, I think it effectively locks you out of the Amex Bonvoy cards for 2 years. You can check this chart if you want details (sorry to link TPG lol).
https://thepointsguy.com/guide/eligibility-chart-marriott-bonvoy-cards/
Only Amex has a Bonvoy business card though, so if you decide to go for business cards, maybe you'd rather go down the Amex track. The bonus on that isn't currently elevated though, so I wouldn't do that now.
When you upgrade a card with Amex, you lock yourself out of the bonus. It would be better to apply for the Delta Plat than to upgrade, but the bonus isn't elevated at the moment. Amex bonuses are usually once per "lifetime", so it's best to catch them at the highest possible.