Really sorry about your DP :(. What a shame although at least you're seeing it now, not years down the line (trying to flip to the positives here!)
A bit of a jumble of info and thoughts below but hopefully it'll help.....
I am really not medical at all, but just based on my own reading / research and experiences.... I think it's hard/impossible to tell whether it is cancer or a fibroid or something else just with a scan. My adenomyosis wasn't detected in the MRI, and the fibroid position wasn't noted, it wasn't noted or seen that the bladder was stretched over it, it wasn't noted or seen that it was embedded into other organs and caused some issue getting it out, and they couldn't confirm 100pc until post-operative lab work that it was a benign fibroid and not something else. Plus if you think about it - if you have an MRI it'll just say "yep there's a big thing there" and you'll still need the surgery. And all you've done is delay it, which if it does need to come out quickly, isn't a good thing to do.
They can feel a really surprising amount of stuff just by prodding your tummy - I'm always amazed. To me it's just a soft squishy blobby tummy! Even the GP (I mean no disrespect to GPs but just making the point it's not a specialist) could feel my "bulky uterus", let alone someone who's primary job is to investigate and treat things in this area, and does it all day in and out.
So if they can't take a biopsy, but they can feel there is something there and they know what size it is, I would be reassured in a way that they're treating it quickly, and not passing you off and ignoring it.
So logically.... someone skilled has felt something large and can tell roughly how big it is. They can't biopsy it to rule cancer out (it could be any number of things - fibroids like you say), so they've decided to whip it out quickly just in case.
I had cancer investigations for something separate about 18 months ago and I learnt that a way to think of the "urgent cancer pathway" is more often than not, about ruling out cancer - not ruling it in. If that helps you at all.
My fibroid was smaller - about the size of a large plum/peach - plus in an awkward place plus other smaller fibroids. But even just the plum/peach was too big to get out vaginally, so a grapefruit would be.
Also think - when they went into get mine, they discovered the fibroid was embedded into other organs and needed quite a bit of dissecting and additional internal stitches and stuff to happen before they could get it and the uterus out, to the point the surgery took 2-3 times longer than the minimum length that op can take. All easier to do with an open surgery and space to move.
Plus - if it is cancer then the quicker they get it out the better (a friend of a friend recently was diagnosed with ovarian cancer which was only discovered during a scan for something else - they couldn't tell if it was cancer until they operated. It was, they did a hysterectomy, and she doesn't need any follow up treatment - the surgery got it all). If it isn't cancer, then yay and you've got rid of something that shouldn't be there and could cause problems and grow.