Here's another perspective: YANBU, the rental agency are.
We were in exactly this position in 2021. We were leaving London for the Midlands, we had 20k in the bank, credit ratings above 800 each (it goes up to 1000), 85k PA income, no kids, no debts, a solid landlord reference and my OH had been employed by his employer for the last five years (which is a long time in IT).
The agency - after asking for six months' rent upfront, then called us back and said 'can you also provide a rent guarantor as well?'
Err, you want me to crawl around my mates asking for favours after the above? Absolutely bloody not.
We didn't. No other agency before or since has ever asked us for such a ridiculous amount of financial safeguarding.
To note: this ONLY happened during covid, when people were losing their jobs left, right and centre.
Tell the agency to piss off. Find a normal rental with a normal agent and a not completely crazy landlord.
As an aside, the agent later called us back and said 'do you know x person, who used to live in the rental you're applying for?'
Huh? We're moving three counties away. Don't know a single person there.
I think the LL had previously had a rent and run or something.
Some of them are just batshit crazy protective (or have very poor or no landlord's insurance whatsoever, which is almost certainly the case with your prospective LL).
We're three more rentals down since the above, and not one single agent has ever asked us for anything as unreasonable since.*
(and yes, I absolutely would be a guarantor for a trusted friend if I actually owned a home, which I do not).
*three months unedited bank statements and full credit reports from us, not through a credit reference agency - no actual extra money nor guarantor