The mountpoint is readonly - thus the data becomes readonly.
/mnt is a system path - and the permissions on the system path is inherited and in effect for your data provided by the share.
It is not recommended to use a system folder to mount your network locations - because you need to set correct permissions.
The article mention this and recommends to create a designated folder structure for your data. e.g.
sudo mkdir /data
sudo chown $USER:$USER /data
mkdir /data/<share1>
mkdir /data/<share2>
Then use those as mountpoints for your shares.
As for unmounting when idle - you can change the timeout in the automount unit
Another approach for mounting is using gio directly - gio is what gvfs provides and pcmanfm uses.
→ [root tip] [Utility Script] GIO mount samba share
One member used the above example to make this tiny GUI for samba shares
→ Need help for mounting smb share with Python - #4 by Golfy