What diagnostic tool would you recommend

I've written it before and I'll write it again. Be very, very aware that you can break things with these tools. You can and should make backups and notes of exactly what you changed when you do so. You will need to understand some of the implications of what you're up to - the names are labelled frequently in German ("standlicht", "Abblendlicht", "lichtfunktion") and following instructions accurately is important. If it says byte 4, it means byte 4 and not 3 and / or 5. No-one knows what changing those other bits will do on your car with its specific combination of control boxes. 

Experiences with FL models and OBD11 are mixed - there were an awful lot of changes in the FL and not all of those seem to have been reflected accurately in the instructions / statements on the app. Note, for example, Sasha's upgrades on the door modules and how some appear to work for some people, others don't.

It's all at your own risk.

For small stuff I would personally suggest VCDS Lite on a laptop with an Autodia cable. I currently have a licence for CarPort and find it to be more expensive and less functional, so I will pick VCDS up at some point when I need it again (which will probably be next week). For fault code reading, you will not even need a full licence, so it's the cost of the cable.

I would also avoid leaving a BT reader in the OBD port; I consider it a security risk.