Wait for shiny Magikarp or evolve?

Your average shiny has roughly a 1/450 chance of appearing. The odds of finding another in a short amount of time are slim. I don't know how far along you are on Mew but if you're close to the Magikarp step I'd just wait.

Honestly I would just evolve. I know the red gyarados is sick but its unlikely that you'll find it anytime soon and if you it could easily be 45 cp and with possibly bad IVs where as you could already have one that is 220 cp with good IVs so i'd personally evolve right away

As long as the Mew quest is available, Magikarp will always return to Raids after special events. Any of the Pokémon required for the Special Quests will still be available in some way. There will be many ways to get a Magikarp going forward.

As for waiting for a Shiny, I don't advise that - mostly because there's literally no way to know when you'll get a Shiny. I've caught about 200 Magikarp since the Shiny was released, and I did manage to catch 1 Shiny. My wife has caught about 300, and no Shiny. You might catch a Shiny on the very next Magikarp you catch - or you might NEVER catch a Shiny Magikarp.

Ever.

If you've got a good Magikarp, evolve it. Get the second move on it (it's only 10,000 Stardust and 25 Candies) and you'll have a very useful Pokémon. And if you happen to catch a Shiny Magikarp after? Evolve it in the future to get the Red Monster.

I’ve been playing 6 months and it took me forEVER to get enough candy to evolve my first Gyarados which is a very useful Pokémon to have. I now have four of them and still no shiny. I might advise to wait until you find a Magikarp better than a pretty good one because you will be happier to have that than you will be to have Mew.

Evolve. Magikarp takes 1km for candy and use a pineapp on every magikarp you catch. I have over 1500 magikarp candies and out of all the ones I've caught, I've only gotten one skiny and it wasn't even good IV's. Evolve.

The best option would be to TRADE for a shiny. Long-time players should have multiple ones and probably are willing to give one away. This way you know what level you are going to get, IVs are better than average, and you even have a chance to get a lucky shiny. Assume you catch a wild shiny after waiting a month .... and he is level 12 with 20% IV. Chances for this to happen are pretty high.