Hi! I'm new here, too! As a reader, I think the most important consideration of POV or modality shifts in novels is how they can serve to enrich your story. As long as your journal entries provide readers with something that you can communicate more effectively or in a more meaningful or impactful way, then go for it!
Are your journal entries written by the same character as your first person narration? If so, this can provide some space for introspection where this character might reveal, allude, or foreshadow something they might not otherwise. Are the journal entries sprinkled in here or there, or is it the other way around? In either case, write what feels right. You can always go back and change things if you don't like it. Just ask yourself what purpose the shift in narration accomplishes, if it adds to character/ story development, and why this specific shift allows that. Answering that last question will clue you in to your character's deeper motivations.
I'm drawing a blank on example texts, but have you read Shade's Children by Garth Nix (1997)? He does a great job shifting between character viewpoints and interspersing them with recordings of journal-like entries from the characters where you get a deeper glimpse into their personalities and development. They're short, but they add so much intricacy to the narrative. I'm trying to think of something more fantasy oriented to provide contrast, but I've got nothing. I'll add more if I can remember anything else.
Congrats on your noveling efforts! I hope this input is relevant and beneficial to your deliberations!