I’m sorry that I put everything together, I wanted to step by step explain the process of autodiscovery and describe the shortcomings that arise along the way.
 

1. Zabbix allow many subnets in one discovery rule. As I see, current concept is "one rule = one discoverer process". If add capacious subnet list like "172.16.0.0/22, 10.0.0.0/23, 10.10.10.0/24" into one rule it will be VERY LONG with one discoverer process. Especially if subnets have random missed/down hosts and free address "holes" (timeouts!). As solution I'm clone discovery rule and create many "subnetted" rules.

I suggest Zabbix can do some discovery address preprocessing: if rule have big address space or many subnets, then discoverer can split subnets to /24 (or to count of discoverers), distribute parts to all free discoverers and run parallel.

2. Then Zabbix run Discovery Action rule (typically add host, add to group and link to template).

a) Many hosts are "not servers" and use another interface only. Why Zabbix add default zabbix-agent interface/ip? If discovery rule does not check "Zabbix agent" then we not need agent settings. Or it can be option or discovery action/operation like "add/remove <type> interface". Maybe someone want discover servers via snmp and then monitor it via zabbix-agent.

b) Action/Operation rule does not save user-defined sequence. Zabbix set operations in alphabetical order. Here can be collisions. Example: we need unlink many hosts from old temlate and link to new template. In alphabetical order first step will be "link host to new template". If both old and new templates have items with same name, collisions can occur. I think Actions may not need strong order, but Zabbix must do all "destroy"(unlink,remove host,etc) actions first.

3. After add host, Zabbix need do LLD discovery and create discovered items/triggers. Many items (like model name, firware version, hostname, etc) change rarely. Also some Discovery rule items (like interface list on ethernet switches) change rarely. Thus we set Update interval to 3-24 hours. And ooops, we get that the first check and discovery of such items will be done only 3-24 hours later.

"First check time = update interval" it's very annoing thing in Zabbix over the years. Host should not be "born in 24 hours in parts". Pick up and create it entirely quickly!

People expect Zabbix automatically do "check now", discover and create all items/triggers in short time (less then 10 minutes) after we add host.