Your choice.

Can i free up slots?

You can't really free-up slots though. The only way to get new slots is by upgrading the airport itself.

It's max level

There is also a level 7 one. Should I buy that, since I can make my own routes?

Still, your choice. There are no costs for owning an airport.

Getting it. YOLO. Also are there upgrade costs?

For sure, yes. It becomes more expensive for each upgrade, Level 8, being the most expensive one.

I waited till it became level 8, then bought it. It makes 43,000K/month

Noice