Agreed, this episode was a fine way to wrap up the series, and yes "this far and not farther" is said by Columbo to indicate this. Although in many of the later episodes, Columbo becomes more arrogant and goofy, in this episode he returns to form as the humble, but intelligent sleuth.
Clive Reville was absolutely superb. I can watch the scene where he is telling the story about his arrest at the fundraising gathering over and over. Even though there are a lot of filler scenes (the limericks, the radio show, etc), they are add to the story. I particularly love the limerick that states (approximately)

There once was a man named Finnegan
who broke out of jail to sin again
He broke laws by the dozen
even stole from his cousin
so the jail he broke out of he's in again

(see, I love it so much I memorized it!)
Of course, Finnegan is really Joe Devlin himself who stole from the Irish-Americans who donated money to help the victims but he used to buy weapons.