Yeah, I know that seem strange to have 46. I'll see if I can make it an even 50, at least! 
Discourse is definitely more resource hungry than the previous forum software, which was actually super lightweight. Discourse uses Ruby and Postgres, compared to the previous Kunena software, which uses PHP and MySql. I don't think that's the main reason for the difference in resource usage though. I think it's mainly due to Discourse's overall system design and how it operates. Firstly, it's a fully "modern" and responsive single page web app with endless scroll. Additionally, Discourse also runs a lot of realtime services under the hood to manage users, realtime messaging, etc.
As for customizability, I actually think Discourse is easier to tweak than Kunena. Discourse has a theming and plugin system that is pretty straight-forward to use and customize compared to Kunena. If it doesn't do everything you want, you can integrate your own theme or component straight from your own git repo. I've already done that to fix a few UI issues. For tweaks and changes to the forum's core functions though, that's a little harder because it involves building the whole application from source instead of relying on the readymade container images and official deployment channels. But even with this, the Discourse dev community is pretty active and seem to turn around updates on a near daily basis, so hopefully relying on the official releases will suffice.
I discussed this in another thread. Unfortunately, it's a limitation of the current Discourse platform. Hopefully, that can be improved in the future.