Yes, you need SIP disabled since I believe pmset just writes to a .plist in a kext.
I just upgraded to Big Sur beta 3, and the pmset method doesn't work, and you need to change some parameters in a power setting's kext .plist. I just tested those changes, and it works. I'll put the steps to follow in my next post.
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This is how I fixed the sleeping issues in my Mac Pro 2019. I don't think these sleep issues are hardware related.
The reason why some Mac Pro's don't seem to have this issue eludes me. Perhaps it's just a question of people not noticing it. My Mac Pro is in a very silent office, and I have very sensitives ears 

, so I was actually disappointed when I got the Mac Pro, since I could hear it clearly. Perhaps other people don't notice the fans turn on because of a noisier environment.
Below are the steps I took to solve this issue in Big Sur beta 3:
Before fix:
pmset -g                 
System-wide power settings:
Currently in use:
autorestart          0
Sleep On Power Button 1
hibernatefile        /var/vm/sleepimage
powernap             0
gpuswitch            2
networkoversleep     0
disksleep            10
sleep                10 (sleep prevented by sharingd, mds)
hibernatemode        0
ttyskeepawake        1
displaysleep         10
tcpkeepalive         1
womp                 0
alex@AlexandsPro2019 ~ % sudo pmset -b tcpkeepalive 0
Warning: This option disables TCP Keep Alive mechanism when sytem is sleeping. This will result in some critical features like 'Find My Mac' not to function properly.
alex@AlexandsPro2019 ~ % pmset -g                 
System-wide power settings:
Currently in use:
autorestart          0
Sleep On Power Button 1
hibernatefile        /var/vm/sleepimage
powernap             0
gpuswitch            2
networkoversleep     0
disksleep            10
sleep                10 (sleep prevented by sharingd, mds)
hibernatemode        0
ttyskeepawake        1
displaysleep         10
tcpkeepalive         1
womp                 0
tcpkeepalive still at 1… pmset is not working.
Since pmset doesn’t seem to be working, even with sip turned off, we’ll need to change the plist containing the power settings. In Big Sur this is a bit more convoluted than in Catalina.
Boot in recovery mode, open a terminal and:
csrutil disable
csrutil authenticated-root disable
Reboot.
After reboot, open a terminal:
diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         3.4 TB     disk0s2
   3:       Microsoft Basic Data BOOTCAMP                595.9 GB   disk0s3
/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +3.4 TB     disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            15.5 GB    disk1s1
   2:              APFS Snapshot com.apple.os.update-... 15.5 GB    disk1s1s1
   3:                APFS Volume Macintosh HD - Data     585.7 GB   disk1s2
   4:                APFS Volume Preboot                 383.7 MB   disk1s3
   5:                APFS Volume Recovery                887.8 MB   disk1s4
   6:                APFS Volume VM                      20.5 KB    disk1s5
Mount your live volume as writtable:
diskutil mount disk1s1
Volume Macintosh HD on disk1s1 mounted
ls /Volumes
BOOTCAMP    Macintosh HD    Macintosh HD 1
sudo mount -uw /Volumes/Macintosh\ HD\ 1
Now make the changes to the power setting plist:
cd /Volumes/Macintosh\ HD\ 1\System/Library/Extensions/IOPlatformPluginFamily.kext/Contents/PlugIns/X86PlatformPlugin.kext/Contents/Resources
ioreg -l | grep board-id
    |   "board-id" = <"Mac-27AD2F918AE68F61">
    |     "IOPropertyMatch" = ({"board-id"="Mac-F60DEB81FF30ACF6"},{"board-id"="Mac-7BA5B2D9E42DDD94"},{"board-id"="Mac-27AD2F918AE68F61"})
sudo chmod a+rw Mac-27AD2F918AE68F61.plist
Now, open Mac-27AD2F918AE68F61.plist with a text editor.
Edit the file so that they match these settings:
<key>TCPKeepAliveDuringSleep</key>
<false/>
...
<key>NotificationWake</key>
<false/>
<key>DNDWhileDisplaySleeps</key>
<true/>
Take a snapshot that Big Sur will boot to:
sudo /System/Library/Filesystems/apfs.fs/Contents/Resources/apfs_systemsnapshot -s "SleepIssuesFix" -v /Volumes/Macintosh\ HD\ 1
Attempting creation of snapshot SleepIssuesFix on volume: /Volumes/Macintosh HD 1
Tag that snapshot for boot:
sudo /System/Library/Filesystems/apfs.fs/Contents/Resources/apfs_systemsnapshot -r "SleepIssuesFix" -v /Volumes/Macintosh\ HD\ 1
Attempting tagging of snapshot SleepIssuesFix on volume: /Volumes/Macintosh HD 1
After reboot:
pmset -g
System-wide power settings:
Currently in use:
autorestart          0
Sleep On Power Button 1
hibernatefile        /var/vm/sleepimage
powernap             0
gpuswitch            2
networkoversleep     0
disksleep            10
sleep                10 (sleep prevented by useractivityd, coreaudiod, nsurlsessiond, mds_stores, UserEventAgent, sharingd, mds, apsd)
hibernatemode        0
ttyskeepawake        1
displaysleep         10
womp                 0
diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         3.4 TB     disk0s2
   3:       Microsoft Basic Data BOOTCAMP                595.9 GB   disk0s3
/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +3.4 TB     disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            15.5 GB    disk1s1
   2:              APFS Snapshot SleepIssuesFix          15.5 GB    disk1s1s1
   3:                APFS Volume Macintosh HD - Data     585.7 GB   disk1s2
   4:                APFS Volume Preboot                 383.7 MB   disk1s3
   5:                APFS Volume Recovery                887.8 MB   disk1s4
   6:                APFS Volume VM                      20.5 KB    disk1s