Upon a closer look, it indeed seems that the certificate count limitations apply to the manually requisitioned certificates, not the ACME ones:
> By using ZeroSSL's ACME feature, you will be able to generate an unlimited amount of 90-day SSL certificates at no charge, also supporting multi-domain certificates and wildcards. Each certificate you create will be stored in your ZeroSSL account.
So I guess that's a viable option.
On the other page, https://zerossl.com/features/acme/ it looks like free acme certificates can have a wildcard records (*.example.com).
https://www.sslforfree.com/ claims they use zerossl and support wildcard records.
Also, one of others you mentioned also has a free acme option, but without mentioning wildcards: https://www.ssl.com/how-to/order-free-90-day-ssl-tls-certifi...