On 23/07/2021 17.57, Schindler, Stefan wrote:
  I tried switching to the ED25519 algorithm, but then
home users could
 no longer resolve anything in the zone because the resolvers are too
 old/incomplete.
 So, to stay resolvable I was hoping to work around that with two
 algorithms. 
You'll have to try, I guess.  A zone using only algorithms unsupported
by a validator is defined not to lead to failure. Just to get downgraded
to insecure status.  So I find it hard to assume anything about such
broken clients (well... I hope your zone was not really broken in some way).