Click the “Replace All” twice and it should do what you need.
Unfortunately it seems like MS Word uses a non standard configuration for advanced searches so functions like “positive lookahead” are not available. That’s why you need to replace twice.
What TTS software are you using? Is there a way to add pronunciation guides that will apply across all software, not just word?
This might mean you only need to replace once, but it will add an extra space after acronyms (“PACU” > "P A C U ") and will inject space in mixed case scenarios (“PACUpacu” > “P A C U pacu”).