[SOLVED] Issue using samba 4.9 with Time Machine

TL;DR: Time Machine cannot create a new backup on my shared drive, but can add to an existing backup.

I'm running macOS Catalina and my Time Machine backs up to a Debian 10 server with NetAtalk and Avahi. Since Mavericks macOS has preferred SMB, and given SMB is marginally faster I decided to switch to using SMB for the Time Machine shares. On a fresh AFP share I can start a new Time Machine backup in System Preferences and it will create a new .sparseimage without complaint.

If I use the exact same directory (/usr/local/smb), so same permissions etc, and create a samba share, when Time Machine attempts to create a new backup it give the error: "Time Machine couldn’t complete the backup to SERVER.local. The backup disk image could not be created."

If I first connect to the share with AFP and do the initial backup, I can then connect with SMB and add subsequent incremental backups without error. I thought maybe a permissions issue, but for debugging purposes I have /usr/local/smb set to 0777 and still get the error.

ls -la showing permissions of the share point:

drwxrwxrwx  5 root smbusers 4096 Apr  3 12:35 smb

I find the following possibly helpful error in the log:

Failed to create '/Volumes/.timemachine/SERVER._smb._tcp.local/DDE06691-7411-41DD-8419-24FEFC21CE29/TimeMachine Set A - SMB/8E394711-7E3F-520B-800C-192D4F680177.sparsebundle', results: {
}, error: 13 Permission denied

afp.conf:

[Global]
; Global server settings
vol preset = default_for_all
log file = /var/log/netatalk.log
uam list = uams_dhx2.so,uams_clrtxt.so
save password = no

[default_for_all]
file perm = 0664
directory perm = 0774
cnid scheme = dbd

[Time Machine Set A - AFP]
path = /usr/local/smb
time machine = yes
vol size limit = 4000000

I'm using some smb.conf options suggested in this GitHub: https://gist.github.com/ChloeTigre/4c2022c0d1a281deedba6f7539a2e3ae

smb.conf:

[global]

## Browsing/Identification ###

# Change this to the workgroup/NT-domain name your Samba server will part of
   workgroup = WORKGROUP
   wins support = yes

#### Debugging/Accounting ####

# This tells Samba to use a separate log file for each machine
# that connects
   log file = /var/log/samba/log.%m

# Cap the size of the individual log files (in KiB).
   max log size = 1000

# We want Samba to only log to /var/log/samba/log.{smbd,nmbd}.
# Append syslog@1 if you want important messages to be sent to syslog too.
   logging = file

# Do something sensible when Samba crashes: mail the admin a backtrace
   panic action = /usr/share/samba/panic-action %d

###MacOS compatability stuff
guest account = smbguest
min protocol = SMB2
map acl inherit = yes
vfs objects = catia fruit streams_xattr  
fruit:metadata = netatalk
fruit:model = MacSamba
fruit:posix_rename = yes 
fruit:veto_appledouble = yes

durable handles = yes
kernel oplocks = no
kernel share modes =no
posix locking = no
smb2 leases = yes


#Turned off for testing compatability
#fruit:wipe_intentionally_left_blank_rfork = yes 
#fruit:delete_empty_adfiles = yes 

####### Authentication #######

# Server role. Defines in which mode Samba will operate. Possible
# values are "standalone server", "member server", "classic primary
# domain controller", "classic backup domain controller", "active
# directory domain controller". 
#
# Most people will want "standalone server" or "member server".
# Running as "active directory domain controller" will require first
# running "samba-tool domain provision" to wipe databases and create a
# new domain.
   server role = standalone server

   obey pam restrictions = yes

# This boolean parameter controls whether Samba attempts to sync the Unix
# password with the SMB password when the encrypted SMB password in the
# passdb is changed.
   unix password sync = yes

# For Unix password sync to work on a Debian GNU/Linux system, the following
# parameters must be set (thanks to Ian Kahan <<kahan@informatik.tu-muenchen.de> for
# sending the correct chat script for the passwd program in Debian Sarge).
   passwd program = /usr/bin/passwd %u
   passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .

# This boolean controls whether PAM will be used for password changes
# when requested by an SMB client instead of the program listed in
# 'passwd program'. The default is 'no'.
   pam password change = yes

# This option controls how unsuccessful authentication attempts are mapped
# to anonymous connections
   map to guest = bad user

######Security#######
security = user
valid users = @smbusers
username map = /etc/samba/users.map
guest ok = no

# Allow users who've been granted usershare privileges to create
# public shares, not just authenticated ones
   usershare allow guests = yes

#======================= Share Definitions =======================

[TimeMachine Set A - SMB]
path = /usr/local/smb
comment = SMB Time Machine Destination Set A
browsable = yes
writeable = yes
create mode = 0664     #tried turning this off, no fix
directory mode = 0777  #tried turning this off, no fix
vfs objects = catia fruit streams_xattr
fruit:aapl = yes
fruit:time machine = yes
#guest ok = yes
fruit:time machine max size = 3.9T  #tried turning this off, no fix
inherit acls = yes