Firstly thank you -
After 45 mins of disassembly and cleaning I was the proud new owner of a working grinder -
Nice work. By all accounts the Tranquillo is an excellent (if a little ugly) commercial level grinder. Does your one have a doser or funnel?
I may not truly understand the limitations of the machine I am using (am I effectively trying to choke the machine to get a good pour as it has no OPV?) -
Pretty much. You may have reached the limit of that machine. I just picked up a 2nd hand Sunbeam EM4820 which is somewhat similar to your Breville. I've been quite impressed with the coffee I get from it with a non-pressurized basket, and if I use one of my good grinders it's better. But it's still way below the level of my semi-commercial machine.
So my questions are as follows:
Is it possible to make a reasonable brew with a good(ish) grinder and the cheap breville machine I am using?
Yes. 'ISH' being the key. (See above).
Am I wasting my time with the unpressurised basket and naked converted portafilter trying to get the nice flow seen on many online videos?
That depends. If you're watching nice naked PF extractions from commercial or prosumer machines then you're never going to match that. But the non-pressurized basket is the way to go and your naked PF will give you a real indication of what's happening during extraction. What are you seeing? A photo or video would help.