For context, I work in a tech lead role where I am often making recommendation to my manager regarding the projects coming down the line. The EM has the final say on this, but they respect my opinion on this matter so I don't want to mess this up.
I also have 1:1s with my teammates whom I am responsible for about their career growth goals. My teammates are all the same eng level, but have different levels of competence and desire to learn.
Additionally, I also know it's important for me to leave some room/ambiguity in project investigation stages to allow folks the space to grow, while it's also important for me to take on some challenging work and help our team deliver more quickly. I guess my question is really 2 parts:
My initial reaction is that this is a premature concern. The scope of work for each of the people on your team is important, and you're a very thoughtful TL for thinking about this.
However, the TL should focus on decomposing the project into phases where, with each passing week, it is progressively de-risked. As TL, you can/should also break down the work into high-level workstreams. Then I'd focus on getting buy-in for your plan, and most likely you'll adjust the plan based on feedback from other leads/managers.
After this is done, then I'd focus on figuring out the assignment of people to parts of the project. Present this as more of a conversation rather than an assignment. The lens through which you delegate (or not) should be based on what's best for landing the project, not based on the people.
In a healthy (high-growth) organization, there's more work to be done than there are people to do the work. So your team will naturally orient themselves based on the criteria of what maximizes the combination of their skill level and project impact.