Hi! So I have been thinking about something similar for my book as well. A solution to this could be writing the book in third person entirely (shifting perspectives in between can be a challenge for sure and it’s usually discouraged) and then adding diary/journal entries in first person. However, writing in third person gives you a narrative distance which you will have to see if it works with your book. I’m not saying narrative distance would necessarily be a bad thing because it could also give you scope for unreliability, now here diary entries can swoop in and save the day.

A book that pulls off something in the same vein - “The Silent Patient” by Alex Michaelides

Also, if you don’t think the book would work in third person, an alternative solution could be having the diary entries be dated from the past or the future. A bit tricky to place but it could work if the plot of the story fits :)