I think Fort Sera already offers a bit of income if you complete the entire Fort Sera questline and become its owner. Over time, from donating captives to the fort (especially generic captives with ID between 5-15 because other captives cannot boost the fort power beyond 100) and Fort Sera being given money by the Republic in exchange for troops during random conclude day events, etc, Fort Sera's resources grows and your daily income from them should grow too. I am earning $400+ per day from them now. Not as much or as efficient as other investments though but the Fort's primary purpose is probably boosting your house's faction power, perhaps for future content.
There's also an indirect way that Fort Sera gives you a boost via a random event that only starts to trigger once you donate enough slaves to Fort Sera to reach the minimum threshold for manpower to trigger the event. I forget the threshold but it's something like 150-200 if I remember right. So even if donating slaves to them is not boosting standing with them anymore, it's still worth donating to them. Also once you hit the standing cap with them you can still continue to boost their power via donations which increases their income.
Once you hit that threshold you will start randomly seeing a "Company Ready" event where the Republic draws recruits from Fort Sera (depleting it's manpower somewhat), and in exchange you gain standing with the Republic. Republic Standing has a very strong effect on income once you have enough investments for it's magnifying effect to really make a difference. Unlike most factions, standing for the Republic works more like the Accountant bonus which magnifies whatever income you're already producing.
Directly boosting standing with the Republic via donations is VERY difficult, so that Company Ready event is the best way to boost it generally even if it's purely random.
What I tend to do is start off donating most of my male slaves to the Vikings to boost their standing and then once I have enough money to finish the questline for Fort Sera ($150k for that final donation), I switch and start donating all of my male slaves to Fort Sera. The vikings accept rifles as donations as well so once I switch my slave donations to Fort Sera, the Vikings get Rifles from then on, although they have to compete with House Carossa for them for awhile.