*You aren't losing 16k, that's money that never existed, simply put into your minds by the estate agent's valuation. All this "giving a stranger 16k" is ridiculous.
I hate the housing market, I hate the fact that people think it is their god given right to make money on it. Not you OP, you sound like a breath of fresh air in a sea of selfish money grabbers.*
Completely agree with this.
You've made money with this house already - it has risen in value, you've rented it out for years, and you aren't needing to buy a new property.
I would think it crazy, and unethical, to pull out from committed buyers for a mythical possible 16k, on top of the profit you have already made from this house.
You have a history of being unable to sell this house in the past (presumably because it was priced too high). So you put it back on the market, maybe you get an offer for 16k more, maybe not, maybe you have a local lockdown and everything stalls, maybe your new buyer loses their job, can't get a mortgage, their chain collapses, or house prices reduce. And your current buyer has lost the money they spent on the survey and been sorely disappointed.
And if you do manage to sell it for 16k more - is that a life changing sum of money for you? Worth the extra risk, uncertainty, stress?
Personally once I'd agreed a sale I'd take my eye off the 'market' and count the bird in the hand. A bit different if someone NEEDS the money e.g. to purchase next house which has gone up, but this isn't the case for you.