MYSQL database failover Current status

I have a MySQL (MariaDB 10.2) server with various databases, to which a lot of people have access to. Which means that users can create MyISAM or InnoDB tables/databases. 

Problem

Mentioned MySQL server doesn't have any serious fallback/backup, except daily backups. There used to be simple master-slave replication in use, but after numerious sync problems, it had to be stopped. I guess that most of problems came from mix of MyISAM and InnoDB tables, but I couldn't pinpoint the problem with 100% accuracy.

Question

What is the best way to set-up some kind of replication/fallback, that would work with minimum overwatch?

One of the problems is also that size of databases is around 100GB and downtime is undesirable. If I want to set-up a replication, is there any option to do it without serious downtime?

Cloud solutions aren't an option.
Mixing MyISAM and InnoDB in ordinary Replication is OK.
MyISAM can screw up in any form of replication.
MyISAM is not allowed in the latest HA techniques -- Galera and Group Replication.
Master-Master, but writing to only one at a time, will let you continue to have MyISAM, and give you quick (perhaps one-minute) failover.
MHA is another approach using standard Replication.


In the long run, you really need to choose between HA and MyISAM.