With the free plan I can receive mail to username+somestring@protonmail.com address, but if I reply to a message sent to this address I can’t edit the sender field before switching to a paid plan. But if I switch plans are those sender addresses limited to any numbe? Note, I’m referring strictly to username+somestring (and username+anotherstring) style addresses, not to addresses that would contain different username part. Those are limited, ofc.
Those are aliases, not addresses. I’m on a paid plan and also cannot edit the sender field to be these. That is only for different addresses.
You probably want SimpleLogin. I’d check that out.
Simple login will appear as from the alias when you respond to an email received through that alias. It does not however (IIRC) change the sender field that the user sees from your default.