Biqu B1 SE Plus Extruder gear slipping

I'm brand new to printing and got a Biqu B1 SE Plus as an early Christmas present.
I haven't been able to do a single print as the extruder gear keeps slipping a couple of minutes into a print. If I manually push filament through, it works but I don't want to be doing that over the course of a 2-hour print.

I've tried everything recommended elsewhere;

• Changed printing temps, speed and z-axis offset.
• Tried different filaments.
• Changed the PTFE Tube.
• Cleaned the extruder gear and re-tightened.
• Pre pulled filament from the roll to reduce strain on the feeding gear.
• Cleaned and changed nozzles.
• Completely replaced hotend.
• Checked, loosened and tightened the feeding lever.
• Checked the tension spring.
• Changed plugs and power cables.

I'm completely lost as to what is causing it, I've had the thing for a week and I haven't gotten a single print over 3 layers, and that one is awful.
Any further ideas or something I'm blatantly missing?