Isn't the OP really asking here (not to have their selection of platform wrangled..): "Where should I target my search: ZA only? is there anywhere else worth dropping my request?"
and: "Are there likely providers of solid colo aside from seacom/tinka-net or workonline/ben-net ?"
Teraco run NAPAfrica, which is Africa's largest exchange point by traffic switched - https://www.napafrica.net/. They also have instances of the INX-ZA exchange points, which are JINX (Johannesburg), DINX (Durban) and CINX (Cape Town).
iColo will be deploying an instance of KIXP
(Kenya IXP) - https://www.tespok.co.ke/?page_id=11648 - and
Asteroid - https://www.asteroidhq.com/.
Raxio will also host an instance of the UIXP - https://www.uixp.co.ug/.
From a network standpoint, you have a few options:
These are the main network operators that run
a good deal of network between Europe, Eastern Africa, Southern
Africa and parts of Asia-Pac.
Hope this helps.
Mark.