Your use case

What would you like to do?

manage my announcement room to only allow mods to post messages but anyone to send reactions, like Element Web Announcements and Synapse Announcements.

to do that, I need to set message PL > default, reaction PL <= default and redaction PL <= default

How would you like to achieve it?

add m.reaction to room settings -> permissions

Have you considered any alternatives?

it might be useful to have some presets like "announcement room, users can only react" which sets message = mod, reaction = redact = default

Additional context

element-hq/element-meta#1385 is closely related