I think what's going on is that you forgot to add myuser as a Samba user. So as a result, you're only able to connect as a guest.
Regarding the requirement of changing permissions on the share to g+w, I can't explain it at the moment. In my testing, I'm able to successfully connect as a guest without changing the permissions on the share. But the difference is I'm connecting from an Ubuntu client and not a Windows client. So perhaps there's something with a Windows client that is requiring the g+w.
The fact that you have force user = myuser as a setting should have allowed the Guest user to have write permissions, or at least take on the credentials of myuser. But perhaps you would also need to add the following to the share in smb.conf:
create mask = 0644
directory mask = 0755
Here's a link on Stack Exchange where it looks like someone was having the same issue: How to create a Samba share that is writable from Windows without 777 permissions?
Anyway, to access a Samba share as a valid user and not a Guest, you also have to add that regular user to the list of valid Samba users.
To see a list of current Samba users, enter the following:
sudo pdbedit -L -v
Next, if you already have a regular user on your Ubuntu system such as myuser, then skip the next step. If not, create a new user:
sudo adduser myuser
Then add mysuser as a Samba user:
sudo smbpasswd -a myuser
If you run sudo pdbedit -L -v again, you will see that myuser has been added.
Within your smb.conf, delete guest ok = yes or change it to guest ok = no.
Restart smbd:
sudo systemctl restart smbd
At this point, you should be able to login to your Samba share as myuser with write permissions.
