It is quite high percentage-wise too. 0.25% or 2500 ppm but it being a custodial transaction, I quite understand why the fee is so high.

Good luck with your experiment, achieving a fee rate between 30 - 447 ppm (the median & the average as reported by mempool.space) should be expected when transacting non-custodially. I've heard that some set a feelimit to a ppm they feel comfortable with. Say 100 ppm. Seems reasonable, especially with small payments.