Is it possible for a team to have great WLB, availability of scope, and impact?

Congrats, you seem to have a good bootcamp mentor! They are 100% correct - There are trade-offs. All that being said:

• I do think it's possible to be decent (not great) at all 3. So imagine a team that's 6/10 in all 3 categories. I don't think it's possible to be a 9/10 in all 3 categories.
• High impact usually comes from working on a high-stakes product. This means that there's more stress, especially during oncall. This makes it hard to have good WLB on a team with very high impact. My org was an example of this - Ads is very high-impact for Meta as virtual ads are 97% of company revenue. However, ads orgs at Meta are known for having poor WLB (mine included). I was able to "hack" a lot of this by being on mobile instead of back-end, but it still took me a lot of work to find good WLB.
• It's true that not enough headcount can lead to more impact/scope, but this will generally cost WLB as there's not enough people to accomplish goals/maintain the system. Not enough people can also hurt E4s going for E5 in particular as E5s need to work through others. You might end up with nobody else on the team to mentor/work through.