It's just the worst isn't it. I've had so many days and nights of tears over my boy, it's like the grieving process for losing him has started now.
I guess the potential 'what if's' are the way in which the tumour will grow - will it grow 'inwards' and potentially impact on breathing/swallowing or will it grow outwards in which case you are possibly looking at it ulcerating or breaking the skin which would lead to infection. Likewise if there is local lymph involvement, they could grow and cause issues in the neck/throat area.
If it has spread to the organs then the impact it has on these could cause some symptoms e.g. if kidneys become affected the first signs may excessive thirst and urination or the opposite, much reduced urination.
I realise this is a pretty vague answer, not dissimilar probably to what your vet has said.
It may be possible to take a sample of the tumour without anaesthetic and sedation, in which case you would have a better idea of the type of tumour and therefore the expected speed or nature of progression.
I just look at basics: eating, drinking and toileting - is my dog doing all of these things easily/normally and if the answer is yes then I look at his general demeanour; if he is bright, alert and interested in life then we keep going. If he becomes quiet, lethargic or withdrawn, or begins to eat less/show less enthusiasm or there are any issues with his toileting then I will reassess. When he was on the higher dose of steroids he had a lot of side effects and I thought our time was going to be very limited, but reducing the dose slightly has really made a positive difference and so we carry on. It is terribly sad assessing each day though with the knowledge that sometime soon I will have to make a call, but I try and look at the positive that he won't have to suffer, he can end his life still feeling relatively good and happy, filled with pizza (his favourite) and having belly scratches while he falls asleep painlessly ❤️