This is ... annoying, I have exactly the same problem.

I want to book MUC-SAN on LH466, google flights prices this through United, but only in basic economy. When I search on united.com for the same routing, it only offers me UA8918 operated by LH, but for 20% more (with the option to buy up to standard economy for €130). Basic Economy to standard economy upgrade are unfortunately only available in the Americas, not Europe.

I'd much rather book with UA in standard economy, but the only way to get google flights to put me on the LH flight in standard economy is to book a hidden city return XXX-MUC-NUE, and not take the bus up to Nuremberg.

Couple questions:
* Anyone know a way to force google to offer the upsell to fully refundable or business? It randomly appears for some flights, but not with any constancy.
* Is there a way to force UA.com to offer an operating carrier flight numbers instead of the codeshare?

Worst case I guess I can book through Expedia, they offer an "upgrade" to standard economy for the same price as UA/LH, but I don't want to deal with them when things do wrong.