Welcome back!
If you do Player Owned Ports, the superior ports armours - tetsu, seasinger & death lotus should do well for slayer and barrows. Chaotic weapons, bought with Dungeoneering points, should get you started.
These are more of a time investment than a gp investment.
Additional resources for returning players:
RuneScape Rewind & more!
has some videos made specifically to help returning players catch up on some of the major changes in the game (from 2014)
More recently, there are RuneScape Recap videos at
https://www.youtube.com/watch?v=Wo2BKJ6zzEE
and
https://www.youtube.com/watch?v=Vb19KRIlfLQ&t
There is also a 'Guide for returning players' on the wiki:
https://runescape.wiki/w/Guide_for_returning_players
As well as some good player made videos:
"Complete Runescape 3 recap (2013-2020)"
https://www.youtube.com/watch?v=KqkDPafdcQM
"10 Tips New And Returning Runescape 3 Players Need To Do"
https://www.youtube.com/watch?v=J-p3tkCxDUY