MrFantastic

Not sure off the top of my head of there is a plugin which allows you to do that, but I could be wrong, there are a lot of plugins.

If you want to know if you "have" to upgrade or not look at IPS release notes, if there is security issues within a release you'll see a warning icon next to the release. https://invisioncommunity.com/release-notes/ I'm running 4.3.6 on one site without any security issues. My other site is running 4.4.1 since that is the last "security" release. It doesn't matter if you have a gaming board or a ton of members - the evil bots could care less about that stuff. Best advice, upgrade to be safe because if something goes wrong you're going to be spending a lot of time cleaning up the mess. If you're frustrated with the upgrade process and your themes / plugins - simple, don't customize anything. Then you generally won't have an issue. Accept the fact that if you customize, you're going to have to put in time to upgrade. That's just a fact of life.

Redis and Redis or Mysql and Memcached. Redis is best in my opinion as it takes a lot of the stress off mysql. Using mysql and memcached works too. You might also want to use elastic search. Using Redis / Elastic search - I'm hitting Mysql 10000 Queries per second average.

Are you talking about Flag as Spammer? You do that from the members profile page. If you need to change a users usergroup to "Banned" - I believe, without a plugin, you can only change usergroups from the ACP.

Error 500 doesn't say much. You have to post logs to really see what is going on which you should be able to get since you're able to access the ACP. Did you turn off all your 3rd party apps before upgrading? Are you upgrading a NULL version, Retail version, or licensed version? Sounds like a template issue to me, but I don't know since you really haven't provided any useful information beyond 500 error. I'd test it, but I'm now an inactive member here.