How do I prepare for tech lead's sabbatical and work with higher ups to take responsibility?

These conversations are best done in person (or at least synchronously). I'd start with the lead who is about to go out, and have a conversation: "I want you to be able to peacefully enjoy your break, here are a few areas where I'd like to fill in the gap in your absence, can you help me get the context here?" And generally the actual knowledge transfer should be documented somewhere like a wiki.

Next have a conversation with your manager/directors: "The lead is going out on leave and I want to ensure the team is well-prepared in their absence. Here are the list of areas I chatted with her about, what else should be on this list?"

Finally, if needed (if you think there's a need/it'll help reduce confusion) you can broadcast out to the team with the plan and talk about how things like oncall or feature questions will get handled

This is an interesting situation - My immediate piece of advice is to view this as an opportunity to grow and add more value to the team as opposed to something to be afraid of. I know that this is easier said than done, but having the fearful mentality often contributes to a self-fulfilling prophecy where the lack of confidence makes it harder to fill in the gaps.

Tactically, this is what I recommend:

• Figure out what's expected of you while your TL is gone - Work with your directors/manager to get a clear picture of this. The main question to answers is, Are you taking on all of your TL's responsibilities or just some of them?
  • If it's the former, you should definitely make sure that you will be able to offload some of your current responsibilities. You don't want to just end up with 2 people's worth of work and get burnt out. Offloading can either come from deprioritizing current tasks or having another engineer take them on (it could even be a junior or mid-level engineer that you mentor to take on some of your implementation work).
• Clearly understand your TL's responsibilities - Meet with your TL as much as possible before they leave, so you can "download" their day-to-day and week-to-week. You can either make your 1:1s longer and have more of them (and if you don't have a recurring 1:1, establish one now for the remaining time). Here are some things to understand:
  • What workstreams are they responsible for?
  • What value do they add to their workstreams? Are they just a decision-maker in meetings? Do they code?
  • How are they prioritizing their workstreams? Are some projects and efforts more important than others?
  • What recurring meetings do they have? There's a good chance you want to set up those same meetings, especially if you'll be taking on the majority of your lead's scope.
• Write this all down - I always give this advice, but it's very true here. As you're learning this information, write it all down. Create a document and share it with your directors + lead, so as you add your learnings to it, they can give feedback and add additional information if you're missing anything. Technical writing helps with so many problems, including this one!