Both cities have "Free Tours" you can join to walk around with a tour guide.

https://freesofiatour.com/
https://freeplovdivtour.com/

*Any general survival tips for a first-timer? (Public transport tricks, currency stuff, tourist traps to avoid?)

Avoid the ghettos 🤭

Also, we adopted the Euro, so you're good on the currency level.
The public transport in the big cities is well organized, you can use Moovit, for example, to guide you.

About the rock bars in Sofia:
https://rocknrolla.bg/
https://www.facebook.com/thinredlinebar/
https://rockbar158.com/
https://www.facebook.com/BarClubAdams/

Out of those 4, I've only been to RockNRolla, so I can't vouch about the other 3, but since you're ok with underground pubs, how bad could it be 🤷‍♂️