The Sidey is good for a while. It’s tiny, but very flexible. You can try different jobs without fear of losing a fortune in rebuys/maintenance.
I’d go about it this way: after you know what you want to do with your next ship, pick the ship. Say, if it’s trade mission running, perhaps a Hauler or an Adder. Then save up your cash until you have maybe 5x its purchase price or more. (You can keep the Sidey as backup, or sell it to invest in the new ship.)
You want to have enough money to upgrade the new ship immediately upon purchase, if not A-rating it, then at least a mix of A thru D modules. You don’t want to go from a fully upgraded small ship to a stock E-rated larger ship! It’ll feel like a downgrade, and it’s discouraging. (And don’t forget to have enough for rebuy on the new ship too!)