That really is grossly unfair and highly offensive.
I agree. Just because people support the RF doesn't mean they support every last thing any of them do.
What I do disagree with is your suggestion that dismantling the monarchy would dwarf Brexit.
Let's say we decided to abolish it before the Queen died. (Which we won't, I know this, it's a hypothesis.) We'd probably let it go on, but gradually reduce the structures surrounding it bit by bit until she died.
At the same time, we'd be removing privileges from the outer circle of royals, slimming the whole thing down, possibly accepting that Charles would be head of state for an interim 5 year period after his mother died.
Buck House and Windsor Castle, and the other state owned properties can be removed as residences. They have enough properties of their own that can be utilised during and after transition.
Any head of state function would be retained until we decided on a new head of state.
And so on.
I didn't vote for Brexit but it's the minority voted government who have made it the clusterfuck it is, it needn't have been anything like this, there were many options going forward rather than this scorched earth policy. Likewise, dismantling the monarchy needn't be a mess, either.
We probably need to sort out our elected democracy first, though.