Galera Arbitrator system requirements compared with the actual Galera MariaDB node requirements
Overall I'd say its best to widen the flow control and send/recv windows, and time-outs between nodes.

If you get down to one node, you can use set global wsrep_provider_options="pc.bootstrap=TRUE" to turn it into an active node without garbs. Lacks automation however is more durable overall.

The nature of the DoS might have an impact in the design. If the app server for the site is down is there sufficiently reduce load on the galera node? Or is the network throughput/latency compromised?

Recommending sticking with a 3 node cluster initially. Its a big step up from a one site.