I would like to suggest load balancing within subnets using read replicas, as a priority over load balancing across subnets.
See my more detailed thoughts here with some team acknowledgement of similar thinking: High User Traffic Incident Retrospective - Thursday September 2, 2021 - #34 by lastmjs
Considering that the high traffic incident was mostly because of query calls, read replicas within a single subnet should have really helped (besides the boundary node issues), and it seems to me a much simpler engineering problem to ad-hoc add read replicas to an existing subnet, rather than split up existing subnets.
Subnet load balancing might make sense for when a subnet is storing too much data in memory, or perhaps for update calls. But query calls I think can be handled in this simpler way.