you should try REST Client: https://marketplace.visualstudio.com/items?itemName=humao.re...
Recently switched to it after going through most of the alternatives, because it has a clean UX, works on every OS without install, and has all of the features I need.
I still think Kong did a good job in crafting the product. I started using Insomnia in my previous company 3 years ago and our team loved it.
What happened recently felt a little bit like the Unity fiasco (of course, in a much much smaller scale). Though as a user I would say Kong had taken a bad turn, as a dev I'm still grateful to them for open-sourcing such a fantastic product.
One of the reasons I forked Insomnia was because it uses React & Electron, both of which I'm quite familiar with. And I think many parts of Insomnia are quite nicely written overall (but unfortunately things like nedb that was used is no longer maintained https://github.com/louischatriot/nedb so it's probably wise to swap out at some point)
Other than that, for UI/manual testing I'd recommend Restfox or Bruno and ezy for gRPC. Hurl and Step CI for automated testing
Restfox: https://github.com/flawiddsouza/Restfox
Bruno: https://github.com/usebruno/bruno
ezy: https://github.com/getezy/ezy
Step CI: https://github.com/stepci/stepci
Hurl: https://github.com/Orange-OpenSource/hurl
Upd: added links