I recently did a similar upgrade, and you are supposed to upgrade to python 3 at the 5.2 level before going to Plone 6.
I went from 5.1.6 to 5.2.10 (5.2.10 seems to be the last version that can install python 2.7 as part of the unified installer)
Once at 5.2.10 with python 2.7 I made a second 5.2.10 install that used python 3.8 and followed the steps in this doc to upgrade my zodb database
https://docs.plone.org/manage/upgrading/version_specific_migration/upgrade_zodb_to_python3.html
- Prepare Your Buildout For Migrating The Database To Python 3
- Verify The Integrity of the Database in Python 2
- Migrate Database using zodbupdate
Once at 5.2.10 python3, I tried to go right to Plone 6 but ended up having to go back and upgrade to 5.2.11 first. You are already at this level, so i'm guessing you want to go 5.2.11 with python 2.7 -> 5.2.11 with python 3 -> Plone 6
Just one side note. I decided to stay at 5.2.11 with Python 3.8 for now, as I did not like the changes to the classic interface (mainly album thumbnails missing) and I was not ready to research volto. But it is good to know when I do need to go to v6, that this upgrade will work. I think my plone site started at either v2 or v3, so I ran into a lot of odd issues I had to work around for the python 2 to python 3 upgrade