Got no good answer for you, as I'd like to know myself.
Defending against tourism is rather easy, since you can just DOW any cultural civ sending trade routes, never accept open borders, steal great works, then later kill off/censor rock backs if not outright destroying the cultural civ unless he also has a good tech and army to defend that.
Part of the problem with culture is imo that there are no real strong secondary spill-over effects into other domains.
Like you mention with science, you will eventually get a huge tech lead that can be translated into a concrete military advantage, but with culture you don't really get that apart from corps/armies (which arrive rather late, and are somewhat limited in effect).
Culture does benefit from religion, but that's religion having a spill-over effect on culture, not the other way.
I really wish Firaxis added some spill-over effects from culture that made a cultural player a more potent threat in multiplayer.
For instance through increased loyalty pressure on neighbours if you have a sufficient tourism lead, similar to how Civ 5 had ideologies.
In civ 5 you could wreak some havoc on other civs if they chose the "wrong" ideology compared to the culture leader, as they would eventually get massive negative happiness and potentially a revolution in their lands (massive internal barb spawns, cities stopping production and if they switched to "your" ideology, they lost all the social policies invested into the "wrong" one).
In civ 6 there are however no such spill-over effects, which hurts a cultural strategy in multiplayer as you can't pressure your opponents with all that tourism (apart from warning them that they need to DOW you, whereas peace is what you actually want so it ends up counter-productive).
I hope to see a spill-over effect for culture in civ 6 that can properly "weaponize" tourism, as the multiplayer meta usually comes down to conquest and tech leads, which is somewhat boring (unless someone pulls off a sneaky religious victory as everyone else focuses science).