As far I understand Galera does not scale write performance, best case it does not degrades it, compared to one single server, so we use Galera for HA. (read performance may benefit and it can be load balanced across every nodes)

In Galera the write performance will be the performance of the weakest node In light of that who wants to got more HA than 3-4 nodes? If one or two fails the write performance will not degrade, even it is possible it increasing...

It is not clear for me, if given a particular write transaction, is there any significant load difference on node that actually the client use to write, compared to the load on all other nodes, who also needs to complete synchronously the very same transaction?

I try to decide, that should I have a dedicated node to write (btw it is single point of error, so I should use some infra to react), and only use the others to read, or try to load balance the writes also across all nodes?

How many nodes are optimal? Is it depends on my application read load/write load ratio?