I want to switch stacks, but my manager has concerns.

Like with all situations, it's important to have empathy, and in this scenario, it's understanding your manager.

Putting myself in your manager's shoes, I think their priority is maintaining the stability of the system. It looks like you are a very accomplished senior engineer, so they have some reservations around stability loss if you were to move off and leave a gap.

All that being said, I think the main thing to do here is a crucial senior engineer behavior: Make yourself replaceable. Here are my thoughts here:

• Find someone (or someones) to mentor, so they can take on your current responsibilities. This is ideally a strong mid-level engineer, but if you have the patience, it could be a junior engineer as well.
• Work with your manager to find this person - They may already have someone in mind. This situation is a win-win-win for all parties, and you should communicate this (removing your self angle). It's a win for whoever you mentor as they become more productive. It's a win for your manager as they will have a more productive team member and a more resilient team as you become more replaceable. And of course, it's a win for you as it will enable you to switch stacks smoothly.
• Set up expectations if you can. Try to understand the bar which your manager has in mind to evaluate whether a replacement is suitable enough to do what you're currently doing.

As a senior engineer/tech lead, it's crucial to learn how to work through others. I did this a lot at Meta and Robinhood, and this allowed me to move around pretty freely and work on what I wanted. I hope you're able to find the same cadence eventually!