Upgrade from 2010 not going well, need to merge multiple files

[NoAnthem]

Was an Office 2010 user, but had to move to a new computer and 2010 was no longer available for download, so I installed Office 2021. On previous machine, Outlook had all 5 of my email addresses living happily together in a single .pst file - no problems. Everything looked and worked just as I needed. I exported this pst file thinking it would import and merge into Outlook 21, but obviously not. What I have now is 1 profile, (named "test", which I would also like to change) but with 5 different .ost files, and 5 different email addresses listed in the navigation pane, all with their own inbox and folder structure. I have not found any way to re-combine all this into a single .ost / .pst equivalent.
I have owned and been involved in multiple businesses and had these email addresses a long time, so I am talkiing about tens of thousands of emails and 8GB worth of data. Manual intervention is not realistic.
Thanks for any help, in advance!