I’ve been cruising from Port Gentil, Gabon, to Principe Island on a 28’ sail boat last november 2019
Somme comments below

Port of entry Santo Antonio, Principe
Very small town, very basic supplies, sometimes even no diesel nor fuel (there was no petrol when we were there)
No services for yatchs,
Gas (butane) coming from Gabon, through Sao Tome, and very expensive (about 35 euros 12 kg bottle)

Formalities quite long, and better if you understand Portuguese.. authorities tend to assimilate yachts to commercial vessels.. even if people is friendly, formalities are long and quite costly.
There are 5 or 6 administration to clear:
• Immigration (no visas requirements for EU citizens), but they invoice clearance of yatch 120 € in business hours and 160 € on week ends. Info on http://www.smf.st/taxas.php
• Port authorities
• Health
• Customs
• Tax police (Policia Fiscal)

Total to pay is about 350 Euros.. seems independant on boat size

The island is very green and beautiful
Security is good, no problems.
One bank , haven’t checked if ATM available
Currency is Dobras, but Euros is widely accepted

Got caugh on a fishing net at night when leaving Principe, fishermen let their several hundres meters lenght net drift and sleep on their pirogues tied to on the net extremity. I would recomend doing approches only by day light.
Have hear about piracy issues near Nigerian waters.. and there was a case of certainly piracy out of Libreville, Gabon, in Decembre.. I would not certainly cruise more north than Principe and would keep clear of all coasts