Hi! PWK starts from basics. However you got few boxes already so know how to enumerate, how to get shells, prviesc.
Reread privesc
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During my course I had no time for HTB, but it was not necessary, boxes are pretty the same here and there.
The only topics i think you might be interested in before PWK are:
BOF - https://www.corelan.be/index.php/2009/07/19/exploit-writing-tutorial-part-1-stack-based-overflows/ this part 1 is enough no need to read others. Try to find other vulnerable software with similar vulnerability.
PIVOTING - https://blog.techorganic.com/2012/10/06/introduction-to-pivoting-part-1-ssh/
If you know these topics already - then just go for OSCP )
Aaaaaand of course Ippsec videos which helped me a lot, to understand so many thing, especially how to think )))
Hope i didn’t spoil anything related to PWK?