Perfect Golduck - now what?

It is - as every 100%-pkm for a non-cheater - a trophy. So do what you do with your trophys - transfer it, keep it, pull it to level 30, max it out - whatever you prefer.

Besides being a trophy, It is a good water prestiger at current size. For this application, you can use it as it is.
It will never be surperior to a full-sized vaporeon.

You shouldn't power it up at all. It's still a very unique and special Pokemon due to it's Ivs and perfect moveset but dust can be better spent powering up Vape w/ WG/HP or Gary w/ HP

If you want to use it as a prestiger i wouldn't power it up at all. I have Golduck with same ms and similar cp, use it to prestige against Rhydon.

Its already a perfect CP for prestiging. As a prestiger, IVs are (mostly) irrelevant. Indeed, at the same CP, a lower IV Golduck might actually be better if it had a lower Attack IV relative to Def/HP. So keep it as is, as a trophy and a great water prestiger against Rhydon and the like.

Just don't touch his CP. His a good pristiger...

With gym CP creep, I think the old 1200CP is now the 1400CP (stagnant gyms bottom at 2800+ or more now).

1400 or so CP is great for prestiging and Golduck is a little tanky so good for all the Rhydon's in gyms.

Overall, grass types are better due to having typing for Vaps too (and Rhydon), but most grasses require 125 candies to fully evolve...

I use my Tangela's daily now...