I am not sure about how lightweight they are (but I guess more than WordPress for sure) but on the federated sode of things you have plume (https://joinplu.me/) and writefreely (https://writefreely.org/) that you can selfhost. Not super sure about how much you can customize them.

I can confirm Orion blocks Youtube ads (might need to tweak options). As for youtube app, no need to sideload anything, Yattee is on the app store and on testflight for betas (https://github.com/yattee/yattee/wiki/Installation-Instructions)