In order to better estimate the necessary transactions fees when sending BC I am searching for backward statistics over the last month resp resp last year.
I would just expand on the answers above by saying that statistics on fees over the last month or year are going to be next to useless when estimating what fee to pay now. The mempool fills and empties so frequently that an appropriate fee could go from 1 sat/vbyte to 50 sats/vbyte several times over a single day, and a sudden and unexpected price swing can cause the mempool to rapidly fill up without warning. You could look at the last 6 months of fees and decide that 1 sat/vbyte is sufficient just as the mempool is filling up and your transaction could sit unconfirmed for days or weeks. Similarly, you could look back at a month of high fees and end up paying far more than you need to when the mempool is already emptying.
When choosing a fee to pay (if I'm not happy to just pay 1-2 sats/vbyte and wait patiently as I usually am), then I'll generally look back over the last 6-12 hours at most to see the general trend of the mempool - filling, emptying, or static. That knowledge, combined with the current size and fee of unconfirmed transactions, is enough to make an education decision. Months of data is pointless.