Maybe configuring service such as fail2ban might help. I would also consider locking down mautic in a way that admin/marketer stuff (everything behind login & login itself) would be possible from specific IP (maybe VPN).
As for the public facing part of mautic - configuring rules within fail2ban should help.
I do not mean to step on anyone´s toes here but this should really be a temporary solution if you want to always use https just redirect from http to https (as far as I understood it earlier this can be done on cloudflare level).
I do not use Cloudflare so I cannot test it, but on the link above, there is instruction to turn off redirect from http to https on your web server (apache, nginx) and configure it, in Cloudflare platform.
I would try to follow the Cloudflare docs first and only then resort to some alternative solution.