The animation 15 years ago was much better, this is a problem
No it wasn't.
Episode 309 is one of the best episodes of Enies Lobby, so its hardly fair to compare it to a fairly weak episode of Wano. Episodes 309 and 304(the first time we see gear 3rd in action) were the best episodes of Enies Lobby pretty much. That was the peak of the series animation wise at the time. A more appropriate comparison would be comparing 309 to the current peaks of Wano, which are episodes like 1015 and 1017.
Even episodes that are strong, but hardly top tier like episode 1013 are still notably stronger than the very best episodes of Enies Lobby.
We also know that the general ambition of the series has improved dramatically in Wano because we have staff who have worked on the show for 20 years who say it has significantly improved since Enies Lobby.
Hell, the animator of the Jet Gattling in Enies Lobby, the freaking scene you linked, Katsumi Ishizuka, said himself that the production is far more ambitious now than it was back then. I believe he said that back in Enies Lobby they weren't really as concerned with the quality of the series, which is in stark comparison to the production of the series currently which from some accounts is very competitive in its production.
The production and animation quality only really began to consistently improve in Thriller Bark, which had the addition of a ton of new and talented staff.
It sort of plateaued for a while after Sabaody, and the polish of the series dipped noticably in Dressrosa even if there were still animation highlights at a somewhat similar rate.
The animation quality improved and stablised in the latter half of WCI after the TV special, and improved massively since Wano. Wano has been the most consistently well animated arc of the series with the best animation highlights and animated episodes by a large margin.