is there any possibility to configure backup of only 1 base? Or even better, certain collections in the base?
Yes, you can make a base backup and thereafter you can take incremental of that base backup. Please refer to the official pbm doc here.
Now, regarding the specific namespace (collection) backup in the base, no, it’s not supported it yet. However, if you want to take a backup of specific namespace, you can go with selective backups but it has limitation also, to know more, kindly go through the documentation over here.