I have to agree with you that Outlook hangs quite frequently. As you have already mentioned that it’s the OLM in which your Outlook stores the mails, it is understandable that you are storing them at offline storage. That also explains your mail dependency. Importing mails to Mail application isn’t the easiest of the tasks. The new mail application has reduced importing options to just one- MBOX files, apart from the previously MAC mail exports.
That leaves you with the only option- conversion of your OLM files to Apple mail. One of my friends recently switched from Outlook to Mail.app. He had searched and tried quite a number of tools. I don’t know many such tools, however, he tested a couple of tools for this conversion. He ended up at OLM converter Pro by Gladwev. Export OLM to Apple Mail format with easy steps go through help of https://www.gladwev.com/portfolio/olm-converter-pro/
He was offered great help from the Gladwev support team for the same. As per him, it was the easiest of all the previously tried tools. Wish that helps.