Since you're an inexperienced Elgg developer, I recommend using ready-made solutions, e.g. these plugins:

https://elgg.org/plugins/3265187

https://elgg.org/plugins/3332885

https://elgg.org/plugins/2922596

Check out the documentation for a more in-depth understanding.

It's not possible to add custom params from you seeder class (as that isn't the cli command).

You could extend/overrule the core seeder command to support your additional params. Those would then also be available to all other seeders, but since they don't use it no harm.

Thanks

1- Use PhpMyAdmin tool

2- Some old tips. Note: be careful with this if you don't understand what you're doing.

Since Elgg is using InnoDB tables you can search how to optimize MySQL for that purpose. But a lot depends on your current setup.

If your on shared hosting with the webserver and database server on the same machine, moving the database to it's own dedicated machine will help. If you're already running the database in a cluster with 10 servers than there are different thing you should look at (but than you probably don't need our help ;)

Are you experiencing any performance issues?

If you have an active community moving the search feature out of the database to a search engine will free up a lot of resources for the 'normal' use of the database. You could look into using OpenSearch as a search engine and use this plugin https://elgg.org/plugins/3333727 to connect your Elgg installation to OpenSearch.