That is, the most restrictive mail server determines the possible number of group members…
Theoretically, you can open a mailbox in phplist or other mailing services, but in practice it is unrealistic.
I see one solution: it is necessary to set up a DeltaChat to send messages in batches, but here is the size and how often - it is necessary to investigate the limitations of mail servers. For example, 10 messages at a time, another 10 in X minutes, etc. And it would be good if the first recipient didn’t start answering until everyone got the message.
Or keep all the groups on one server (Delta Chat server), which carries out distribution to all recipients at once. And removes the letter immediately after the distribution, and there are a lot of servers in transit.