So coming back on this bug report.
I have upgraded to 6.9 on March 2nd and also wiped the cache to take advantage of the new partition alignment (I have Samsung EVO's and perhaps a portion of OCD 🤣).
Waited a bit to get historic data.
Pre 6.9
TBW on 19-02-2021 23:57:01 --> 15.9 TB, which is 16313.7 GB.
TBW on 20-02-2021 23:57:01 --> 16.0 TB, which is 16344.9 GB.
TBW on 21-02-2021 23:57:01 --> 16.0 TB, which is 16382.8 GB.
TBW on 22-02-2021 23:57:01 --> 16.0 TB, which is 16419.5 GB.
-> Writes somewhere on 34/35GB's average a day.
6.9
TBW on 05-03-2021 23:57:01 --> 16.6 TB, which is 16947.4 GB.
TBW on 06-03-2021 23:57:01 --> 16.6 TB, which is 16960.2 GB.
TBW on 07-03-2021 23:57:01 --> 16.6 TB, which is 16972.8 GB.
TBW on 08-03-2021 23:57:01 --> 16.6 TB, which is 16985.3 GB.
-> Writes round 12/13GB's a average a day
So I would say 6.9 (and reformatting) made a very big improvement.
I think most of these savings are due to the new partition alignment as I was running docker directly on the cache already and recall made a few tweaks suggested here (adding mount options, cannot remember which exactly).
Thanks @limetech and all other devs for the work put into this.
This bug report is Closed 👍