Yes all the things above work, I have done them all
With the flapper open clean very well usually fixes it
To replace the seal you can get the seal kit from a Demetic place (I used me sanitation supply) for about $85
Make sure you tell them how old (mine is 2002)
Unscrew four hold down screws lift toilet bowl out of way unscrew ring and old seal comes out new seal put in and screw ring back on
Put new foam ring on and reinstall toilet
Will last for years