The Finnair app also sometimes lists cash upgrade options for the flights a week or so in advance.
Obviously you cannot count on this on always being the case, but just another option.
I've seen some pretty inflated upgrade prices on the LHR route recently (think of northwards of 150€) which IMHO isn't worth it even on the A350 but your perception of value may be different to mine and depend on things like status (lounge access, free drink onboard for OWEs etc.).