Short-term rentals in central St. Petersburg are performing worse than a couple of years ago - there are fewer tourist flows, competition among landlords has increased, and platform commissions have stayed the same or gone up. Airbnb takes its percentage, you need to pay taxes (if you're on the standard system, that's roughly 13% of income plus insurance). After all that, your margin shrinks noticeably.
You need decent renovations - tourists are pickier than long-term tenants. At minimum, that's fresh cosmetic repairs, new furniture in the bedroom and kitchen, a nice bathroom, and cleanliness without compromise. You'll need to invest tens of thousands at the bare minimum, possibly more if the apartment is in bad shape. Plus ongoing expenses: cleaning after each guest, utilities, repair costs.
Your actual profit depends on the season and occupancy rate. During tourist season, a one-bedroom in the center can bring in decent income, but the other months are often unprofitable or barely cover expenses. You should honestly calculate: how many days per year will it be occupied in your area given current competition, how much it'll bring per night, and subtract commissions, taxes, and cleaning. If it comes out profitable even in an average month - then it's worth trying.