Finally saw it. Film Gold is fun but it's also maybe my least favorite of the Film trilogy. Overall I'd put it at #5, behind Dead End Adventure and Baron Omatsuri as well.
Tesoro is far and away the least interesting of the Film villains with a backstory that feels very much tacked on instead of shaping his character. He's basically just an arrogant gloating villain with none of the personality or motivation that made Zephyr and Shiki so memorable. His fight, while the most visually stunning of the three climaxes, was also the most bog standard anime of them and frankly the most boring. The Gran Tesoro is very cool and certainly a better setting than the random islands of Film Z but stills doesn't match the bonkers creativity of Strong World. The shenanigans on the ship and the heist later were both my favorite parts.
While I loved the random cameos, the appearances of Spandam, Lucci, and Sabo were all basically just fanservice that did nothing for the movie. Contrast that to the big role Kuzan and Borsalino played in Film Z; even the minor inclusions of Garp and Sengoku served a narrative purpose and didn't distract from the story. That whole small subplot could be cut out to make the movie shorter, or extend more meaningful parts like the heist. Speaking of which, I loved the goofy visual and musical nods to classic movies like Mission Impossible and Ocean's Eleven. I think the movie would have benefited by making their plan more elaborate, explicitly establishing a purpose for those initial disguises, and of course clearly creating an opportunity for Franky to readjust the plumbing that's noticeable in hindsight.
Essentially the movie was fun but missed the mark on the attempted emotional cores, with an excellent setting, great gags, and mostly stellar animation despite a lackluster villain.