I traveled around Equatorial Guinea, Gabon, Congo-Brazzaville, Central African Republic, Sao Tome and Cameroon June-July 2018 if you have any questions. I did quite a bit of chit chat with a couple of people who were planning/did travel in the region and you might find our discussion informative. https://www.lonelyplanet.com/thorntree/topics/our-latest-trip-through-angola-in-29-days/posts/new?reply_to_id=23034440
I also highly recommend buying the Bradt guides for Equatorial Guinea and Gabon (ozymandias24
wrote the Bradt Guide for Equatorial Guinea). Both books will tell you everything you need to know about each country. The Bradt guide for Congo-Brazzaville is part of the Congo-Kinshasa guidebook and doesn't offer very much practical advice at this point though has some interesting discussed history in it (such as the story behind Congo's Ninjas).
I found Gabon easy to travel around, Equatorial Guinea to be complicated, and the Central African Republic to be difficult. Stick to the Southwest of CAR and you should be fine. I know of a number of "country counters" who fly in and out of Bangui only (with maybe a side trip to Mbaiki and Baoli falls) which is probably your easiest option and then most other tourists I think take the regular charter flight from Bangui to Bayanga and stay at the Sangha Lodge (easy to do but expensive... which I would like to have done were I more wealthy). You have the option of entering CAR by land(on really shitty roads)/river crossing from Cameroon and river from Congo-Brazzaville (pay a pirogue in Bomassa to take you upriver. Both those options I would dub adventurous.
Most people visiting Equatorial Guinea enter by flying into Malabo. I entered and exited by land through Ebebiyin (to the continental part of the country called Rio Muní) the only land border at that time letting tourists through (was turned away/kept in at other borders). I haven't been to Bioko but seeing the Djibloho hotel and Oyala was interesting. In fact everything in Equatorial Guinea was interesting. Strangest place I've ever been. Their doctors are from North Korea for example.
