[h_anna_h]

The three first clients in the joinmastodon site:

Tusky: https://github.com/tuskyapp/Tusky

Fedilab: https://framagit.org/tom79/fedilab/

SubwayTooter: https://github.com/tateisu/SubwayTooter

All of them are foss and developed by a single person or a small team.

As for the system requirements. Mastodon might be a bit heavy but you could try another server software with activitypub support.