I'm using Raycast, not ChatGPT, but I think you can simply create custom GPTs for each use case (with prompt as a system instruction) and then use @ in the default new chat to quickly switch to the desired bot.
In my opinion, the results from Kagi Assistants are significantly inferior. I'm subscribed to both services and tried the Ultimate tier for a few days two weeks ago to test the new assistants. If they add the ability to search with multiple steps and allow the AI to digest more results, then they could be on par. It's more similar to ChatGPT at this stage, IMO.
If you prefer using GPT-3.5 due to its lower price or speed, wouldn't it be better to switch to Haiku? People were even able to match the performance of Opus when they added a couple of examples to the prompt.
"Unfortunately, Claude.ai is only available in certain regions right now."
GPT 3.5 used to be good enough, so I never bothered getting a paid account. I also heard some reports about 3.5 actually being better for the type of coding tasks I usually offload.