Gemma, first: 21 eggs with a high oestrogen level is precisely the picture where a freeze all is the textbook safe call, so your team are doing the careful thing, not the disappointing one. Let me answer both your worries directly and then give you the shape of it.
On chances: freezing all your embryos does not, for most people, mean giving something up. Modern freezing (vitrification) means the large majority of good embryos survive thawing, and frozen transfer success is now broadly comparable to fresh. What predicts success most is the age of the egg when it was collected, which a freeze all preserves exactly. So you are banking this cycle's work, not spending it.
On OHSS: ovarian hyperstimulation syndrome is an over-response to the stimulation drugs, and it is driven by the hCG in the trigger and, crucially, by the hCG of an early pregnancy. That second part is the whole reason for the freeze all: transferring fresh and becoming pregnant now would pour fuel on exactly the fire they're trying to avoid, so they delay transfer until your ovaries have quietened. Your risk markers are the classic ones: a high egg number (above 15 to 20 is a common threshold), high oestrogen, and often a high AMH or PCOS behind it. For scale, mild OHSS bloating like yours is common, up to around a third of cycles, while the moderate to severe forms are much less frequent, roughly 1 to 5 in 100, and a freeze all plus close monitoring is one of the most effective ways to keep you at the mild end. Our guide to OHSS sets out the full picture, and frozen embryo transfer explains what the next step actually involves.
The exception worth holding clearly: mild bloating that is slowly improving is expected and fine to manage at home with rest and fluids. But rapid weight gain, severe or worsening abdominal pain, breathlessness, or passing much less urine are not wait-and-see symptoms, they need same-day contact with your clinic. If you're ever unsure which side of that line a symptom sits, that uncertainty is itself the reason to call. What your own team can tell you that this thread can't is the plan for your specific FET timing, which is the right question for your follow-up.