2013 Škoda Citigo Sport sound upgrade

Good evening all, I trust that you are all doing well on the last day of the year.

I would like to ask whether, given that I wish to upgrade my sound system to something a little bit better, I should upgrade my head unit or just go straight to upgrading the speakers?

My main issue is that the higher pitches aren't as good as the bass, and they are a little bit screechy.

I currently have a Bluetooth AUX dongle, which connects to USB power and the AUX, and connects to my phone via Bluetooth when the ignition is turned on.

This works a treat, it's just the quality that could be improved.

TLDR: Upgrade speakers, or does the head unit need doing first? If so, recommendations on a budget?