Measurement/dialling Breville the Barista Express

Hi everyone, I’m pretty new to properly dialling in espresso and I’m struggling to understand what exactly is going wrong with my Breville Barista Express.

Current setup:
• Grind size 5
• 15g dose AFTER tamping/levelling excess off
• First drip starts around 9 to 10 sec after pressing the button
• Total extraction finishes around 22 sec
• Yield is around 39 to 40g espresso out

A few things confusing me:

1. I’ve read the pressure gauge is inaccurate and should mostly be ignored?

2. Apparently the machine has a ~10 sec pre infusion, so does that mean my “22 sec extraction” is actually much shorter?

3. I physically can’t seem to fit more than ~15g in the double basket after tamping and levelling

4. The automatic double shot button seems to keep pushing ~40g liquid regardless of grind changes

5. Espresso shots dont seem hot enough, despite changing the programming to +2 degrees

I’m trying to understand whether my issue is:
• grind size
• underdosing
• channeling
• the auto shot programming
• or just misunderstanding how this machine works

Would appreciate advice! Thank you