I'm not convinced that a holding company is going to happen. Today, there's a great deal of collaboration and cross pollination between Tesla, SpaceX, and Boring, and this will continue regardless.
To comply with US defense-related rules, SpaceX needs to be kept reasonably independent of Elon's other companies.
For SpaceX, the main reason to come under the umbrella of a holding company would be to eliminate the need to continue to seek additional, outside funding. However, this is hoped to be a short term problem, assuming Starlink ends up becoming a cash cow. Even now, SpaceX has a high valuation on its own, and raising funds through a Starlink IPO will be another option.
Perhaps a holding company, with Tesla, might make more sense for Boring and Neuralink. Even if Tesla were to acquire those smaller ventures outright, it wouldn't cost the shareholders much, though Tesla would undoubtedly be accused of "losing focus".
Elon can also sell more Tesla shares and personally invest the proceeds in SpaceX, Boring, and Neuralink. The downside to this approach is that some of the funds would be "lost" to taxation. However, that might not be all bad, as continuing to pay significant taxes would help Elon to push back whenever the political Left accuses him of not paying his "fair share".