Unlike in the cartoons, bellies of large creatures, such as whales and dragons, don't necessarily have chambers of breathable air in them.
So you should probably work on some acid resistance and find a way to become the fish that gets swallowed. That eliminates the "can you teleport inside" issue. You'll still need a way to move/breath inside. I leave that to you, since you know what you can actually gain access to.
Typically swallowed creatures are treated as "grappled" for purposes of fighting their way out. So Still Spell is definitely on the list of things you'll want, unless you can get that rare
Ring of Freedom of Movement.
Because of the limited space inside,
Summon Swarm probably isn't a workable tactic.
Consider
Shrink Item as your next best approach. So much cheaper tha the actual magic item
Folding Boat. Shrink a number of dangerous things, pointy things, uncomfortable things. Barrels of caltrops with eight inch iron spikes sticking out of them. 10 foot long-spears with iron hafts. If nothing else, think heavy: How well can a dragon fly with a few thousand pounds of iron and lead shot in his belly? Pieces too pointy and oddly shaped to pass without 50 gallons of Preparation H.
We generally presume that anything under
Shrink Item can only expand if there's room to expand, so don't go overboard. You want "uncomfortably large", not "pop him like a balloon".
And just remember that you can throw down a whole bunch of items in the same round, if you do it by the handful, so if you time it for a moment in the air you can ruin his whole day.
Also consider some form of
Fire Immunity and a
Necklace of Missiles. Take a missile, throw down the necklace, and detonate the missile. All the others will go, and the Dragon will have a case of heartburn to end all heartburn.
Then save that
Teleport for your escape.