Pick 2 suitable fleet groups. Get planes. Fly them on efficient schedules. Make more money to get more planes. Repeat.
That's pretty much it, regardless of where you're based.
You've got 1 Heron, and you have quite a lot of routes inside the Heron's range, 10-50 pax, and empty or nearly empty. So get more Herons for those. They're cheap, they're also available new.
You've got plenty of bigger routes, but many of them are too long for a turboprop. So pick a fleet that's readily available and long lasting. Compare price, availability, maintenance costs, fuel costs. Some of your options are
Comet - very thirsty (7000 kg/hr), a bit expensive on maintenance (~30k/month for the 8 year old stuff on the market)
DC6 - cheap to lease, not too bad on fuel. But the planes are old & slow, and maintenance is very expensive (30k/month to lease one, but over 50k/month to maintain it)
IL-18 - the newer 18Ds are decent, but as a group they're also somewhat slow, and expensive to maintain.
Tu-104 - similar to the comet. 7000kg/hr, a little cheaper to lease, but a little more to maintain.
Caravelle - smaller than the others, which isn't automatically a bad thing. Better on fuel, at 4500kg/hr. Lease + maintenance is about 80k/month.
737 - better again on fuel, at 3500kg/hr. Also about 80k/month for lease + maintenance. Shares a fleet group with the 737-200, so you can replace with those later on as demand grows and the newer planes arrive.
Personally, I'd go with a mix of 737s & Herons. If you want an estimate for what a plane will cost you in maintenance, take the price it shows for A-check, and multiply by 11. That'll give you the average price/month, with yearly C-checks included. Lease price/month you can see on the plane's page already. Cheaper leases are not always better. Even though the DC6 is half the price of a 737, it will actually be more expensive to run month to month.