I think thats exactly the opposite... fee rate vs time is not very useful:  Time just adds in the noise of a possion process with no influence by fees.

If you have a good estimator based on block count you can simply multiply it's predictions by the exponential distribution of interblock intervals and get a good estimator of time... but to take data that has been smeared by random block times and construct good estimates is hard due to all the injected noise.

In effect, your time data is heavily biased by random correlations with higher and lower fee intervals with luckier or less lucky block finding.

This remains true so long as people aren't turning hashrate on and off based on fees-- and so far as I know it, no one is today.

An interesting chart is a grid over n-blocks-wait and fee-rate,  then for each cell set a value of what percentage of transactions paying at least that fee rate were confirmed in at that number or fewer blocks.

How does your data handle transaction replacement?   How do you compute feerates for CPFP transactions?   One possibility is to only consider transactions which are not dependent on unconfirmed transactions and which have no children;  and similarly do not consider replacement or replaced transactions.