Understood.
Okay I did some more testing this time with an exFAT volume, and am successfully writing individual files with the below global settings.
You need to UNCHECK Enable macOS compatible shares from Luci Samba4 General settings page - this checkbox attempts to force fruit:metadata = stream , which according to this post causes issues with exFAT. Then force fruit:resource = file instead of stream.
I also removed streams_xattr from vfs objects so that xattribute support isn’t even loaded globally.
Below is my global template.
vfs objects = catia fruit
fruit:resource = file
fruit:nfs_aces = no
fruit:model = TimeCapsule
fruit:wipe_intentionally_left_blank_rfork = yes
fruit:delete_empty_adfiles = yes
And per share, yellow highlighted is exFAT.
As per aforementioned post link, I changed the mask values, maybe its not necessary.
Create mask: 0744
Directory mask: 0755
Save in Luci, and then in cli
service samba4 restart
or reboot router.