Do I have to erase a stolen iPhone on lost mode in order to protect my data?

About a week ago, two thieves stole my iPhone out of my pocket while I was distracted with my toddler. I realized about a minute too late but by then, the phone was already switched off.

I activated Lost Mode not long after I got home and my phone already had a not-so-simple passcode on it so I wasn't too worried about my private data. But mainly, I haven't backed up my phone in a couple years so I have quite a lot of pictures of my kid as well as voice memos that I'd used to record my kid's babbles ever since he was a baby that I desperately want to recover.

Today, Find My iPhone finally alerted me that my phone was turned on. And I am feeling quite worried because I do have a lot of personal data on the phone. The Internet seems to say conflicting things about Lost Mode - some sites they it's very secure and the thieves won't be able to ever get into my personal stuff (and if that's the case, I'd rather not erase my phone in hopes of one day, maybe getting the pictures back) while other websites seem to claim that Lost Mode can be deactivated or bypassed, which makes me extremely nervous about leaving the phone un-erased.

Can anyone from Apple or highly knowledgable about Apple's security features please weigh in on this? Is it risky to un-erase my phone? Can the thieves get past a passcode and Lost Mode? Thanks in advance.