Write cache / delay as long as possible

I am trying to setup Linux to delay the writes as much as possible (minutes). To those wondering why: this is Chia plotting. It is very write intensive done in SSD/NVMe, if the system crashes you have to restart from scratch anyways.

Minimizing the writes to disk actually increase the life expectancy for the disks.

That said I setup a testing system (20.04.1-Ubuntu LTS in a VM Hyper V - Windows 10).

I setup my "dirty" flags as:

sudo  sysctl -a | grep dirty
vm.dirty_background_bytes = 0
vm.dirty_background_ratio = 75
vm.dirty_bytes = 0
vm.dirty_expire_centisecs = 120000
vm.dirty_ratio = 80
vm.dirty_writeback_centisecs = 180000
vm.dirtytime_expire_seconds = 43200

That gives me about 3GB of cache (I am using 4GB ram for this VM).

I am following these instructions to test, changed to this (so it is random and 1GB)

 dd if=/dev/random of=testfile.txt bs=1M count=1000

And using

cat /proc/meminfo | grep Dirty
cat /proc/vmstat | egrep "dirty|writeback"

To see whether the number of dirty pages are increasing as it makes progress.

The number of dirty pages increases as expected until dd finishes. When it finishes the number of dirty pages goes almost instantaneously to 0 and Windows starts writing the data (I am monitoring the Windows side using the Resource Monitor to see when the writes actually hit the disk).

So... is it because Linux flush the file as soon as it is closed by dd? maybe it is dd doing the flushing? I was expecting the file to remain in the dirty pages as it is smaller than the available space for that (this testing system is not doing anything else).

Any ideas? Thanks.