Galera maxes out at somewhere around 5 nodes in the cluster. Probably it is due to the broadcasting of each write to every other node, and waiting for replies.
There are many ways to scale up a MySQL; Galera is one of them, and it is perhaps the best today for write scaling.
For read scaling, replication slaves provide virtually unlimited scaling. You can hang traditional replication slaves off of each Galera node. This will let you offload the reads from your 5 nodes. Slaves can be cascaded (using "relays"), thereby giving unlimited scaling. A server can easily have 10 slaves hanging off it; do 6 levels of that, and you have a million servers. (I have not worked with more than 3 levels and 30+ slaves.)
A common way of scaling up is to look at the code. Composite indexes is unknown to a lot of novices. For inserting, batching and LOAD DATA
is very effective. For Data Warehousing, Summary tables can often speed up "reports" 10-fold. For high speed ingestion, ping-ponging a staging table is very good. For GUID/UUID indexes, abandoning them is best. Ditto for EAV. For huge deletes there are several approaches.
And my tips on Galera.