If you already installed bind9 with Hestia, there is no need to remove it and install unbound, you can just use bind9 as your dns resolver. Also, if you remove bind9 you won’t be able to add dns records to your domains (don’t know whether you are using it for this purpose).
Yes you should modify that file with that conf but keep in mind that it could be overwritten by your system if you are using systemd-resolved… so you should do more steps to change the nameserver to be used. Take a look to this post: Not receiving emails in roundcube - #16 by sahsanu
If you prefer to install unbound and you will use bind9 too, you can configure unbound to listen on address 127.0.0.2 and add nameserver 127.0.0.2 to your resolv.conf file so there isn’t need to remove bind9. You should add this directive listen-on port 53 { 127.0.0.1/32; }; to /etc/bind/named.conf.options so bind will listen only on 127.0.0.1.