Internet is fine. The Canaries are a staging point for several transatlantic fibres. There is also an extremely active astrophysics group on both Tenerife and La Palma who shunt a lot of data around the world. You'll pay more for it, but that's because Spanish telcos are a monopoly. You might not get 100Mbit, but certainly 20M+ is normal.

Tourism isn't really a problem because the tourists stick to the resorts. The upside of a flourishing tourist industry is that you can get dirt cheap flights from all over Europe.

I've spent a lot of time in Tenerife. Santa Cruz, in the North, is a lovely city. The weather is pleasant all year round, beaches nearby, the people are friendly and the food is excellent. Accommodation is cheap and readily (and informally) available, prices are generally lower than mainland Spain so for most Westerners the cost of living is low. Most amenities are there - you've got a cinema, large supermarkets, big brand stores and so on. Getting things online may take a bit longer, I missed Amazon Prime.

You will almost certainly need a car if you plan on doing any kind of travel around the islands. Gas is about the same as most of Europe, perhaps a little lower.

The main downside for foreigners is that outside of the resorts fewer people speak English, but Spanish is easy enough to learn. There is a large ex pat community, though they tend to concentrate in the resorts for obvious reasons.

(Also Carnivals are great! Don't listen to the person who said otherwise. They run for a week or two every year, everyone dresses up, gets drunk and pees wherever they can.)