Don't know about Kong Cloud - Not sure it is prod ready / Generally Available yet. No pricing info or ability to purchase kong cloud without speaking with their sales reps.
If you want custom plugins, Tyk does not allow this in their pure SaaS offering. But with Tyk Multi-Cloud https://tyk.io/api-gateway/cloud/#multi-cloud You can write your own custom plugins in Python, Lua, Javascript, or any language supporting gRPC.
The difference between Tyk Cloud and Tyk Multi-Cloud is that Multi-Cloud offers a hybrid cloud agnostic architecture allowing you to deploy your gateways in your own infrastructure (more secure & more performant close to your backend services), with the management layer offered as a SaaS. Tyk Cloud is a pure SaaS solution.