Heya, my wife and I cycled through Mexico for about ten months last year and slow internet pisses me off! I think the Nayarit coast is a good option for you. I see you already are in Sayulita, but had already written about it before rereading your post, so I’ll leave it in here so you can compare what I think of it compared to the other places I mention.
Here are my thoughts:
We spent three weeks in an apartment in San Blas and absolutely loved it. It’s a beautiful place with great surfing (apparently), and is very chill and more local than most of the beaches in the area BUT that is because they have a lot of annoying insects. It was totally worth it for us, but definitely something to be aware of. We stayed at La Quinta California where we were able to rent a nice (owner calls it rustic, but we thought it was very nice) apartment halfway between the center of town and the beach. The owner used to run an internet cafe there and pays extra to have the best internet possible. You can read a little more about our time in San Blas with some pictures of the incredible beach here: https://www.crazyguyonabike.com/doc/page/?o=tS&page_id=446193&v=K8
Just a little further down the coast is the gringo haven, Sayulita and its little brother to the north, San Pancho. Sayulita also is a surfing spot. You won’t have the bugs like in San Blas, but it will be much much more expensive and completely overrun by gringos. I’ve also heard from some windsurfing buddies that the water quality is quite bad because of a sewage outflow right into the bay. The town is really beautiful and there is a ton of great food there. One thing I liked about it despite being a tourist central is that it still has a pretty laid back vibe and all of the Mexicans who work there or own businesses are really cool people who are attracted to the multi-cultural vibe of the place compared to whatever conservative uber religious place they came from. San Pancho to the north has more of a hippie vibe with a really cool sunset scene on the beach, but it’s just as expensive as Sayulita. You’ll definitely find more music and restaurants and stuff happening in Sayulita and Pancho than San Blas, and you’ll be much closer to Puerto Vallarta in case you need to get any supplies you can’t find in a village. All three should have pretty solid internet unless you rent from someplace where the network is being poached.
I suggest you avoid the Oaxaca coast (at least Mazunte, San Augustin and Zipolite) like the plague. These are NOT places for a digi nomad. The internet is some of the worst we’ve seen anywhere in Mexico or Central America and there is a LOT of theft, especially in Mazunte. We were robbed in Mazunte and every day we were there we heard of someone else being robbed. Unlike other places we’ve been the locals don’t care at all. In fact, I really didn’t like the locals that much compared to everywhere else we’ve been in Mexico, especially in Nayarit. In fact, we were amazed at the difference in the attitudes of the locals there compared to the incredible friendliness of the people in the tourist town San Jose del Pacifico in the mountains right above. It’s unfortunate because the traveler vibe on the beach is pretty cool and there is a great mix of local and gringo food.