Movim https://movim.eu/ could serve a dual purpose for you if you don’t yet have chat which often make sense for certain non-tech-savvy circles. It can be an all-in-one application for chat + posts (short form, long, with media, & now whatever those short videos are called)—& this better mimics how many interact with Facebook. It isn’t ActivityPub, but it is FOSS / decentralized (can join or disable connection to other pods/servers). There aren’t chat applications integrated together with ActivityPub—& Mastodon doesn’t provide E2EE.
It also functions as a PWA (progressive web app) for those that want those features. The underlying chat protocol has many native clients as well for everyone to have a low-power, low-data-consumption option for encrypted chat, both direct messages & chat rooms.