No decorations? It seems they all wear the highest order of their own country...
The President of South Africa was wearing the Grand Cross of the Order of the Bath, which seems to be the standard award in the UK for visiting presidents of republics (except those who are barred from receiving foreign orders such as the POTUS).
South Africa has recently changed its honors system. The Apartheid-era Order of the Good Hope has been discontinued and replaced with the Order of the Companions of O. R. Tambo, which doesn't have a riband. The badge is worn, as I understand, around the neck. I don't know if the King has not been awarded a South African order, or if he, for some reason, did not (or could not) wear the insignia. Maybe the South African press has an explanation.
Overall, seeing the video yesterday evening from the banquet, I had the impression that everybody was rather tense in their new roles, in particular Charles, Camilla, and Kate.
The Queen Consort was rather agitated and talking to the page behind her about something before they were seated and it is unclear to me what that was about.
The King's speech was fine, but he rushed a little bit into it and placed an unusal stress in some of his lines changing his voice tone in an odd way. In the first lines, he seemed to be saying "welcome" or, more precisely, "hello" to the President in several South African languages, but failed to say it also in Afrikaans. That might be considered OK by woke standards, but in my opinion is wrong. Afrikaans is not just the language of white Afrikaners. It is the language of more than 12 % of the population of South Africa and has more non-white than white L1 speakers, especially in the Western Cape and the Northern Cape. The language has been under vicious attack from the ANC, which has been shutting down Afrikaans schools and universities across the country or forcing them to switch to English-only instruction, in violation of the South African constitution, which guarantees home language education, and is now an endangered language, as controversially mentioned coincidentally this week by the actress Charlize Theron. I regret that the King failed to acknowledge the language's existence and its status as one of the 11 official languages of South Africa.
Queen Beatrix, by contrast, during her state visit to SA at the time of President Mandela, briefly changing to a few lines in (standard) Dutch, referred to Afrikaans, grouped with Dutch, as "onze taal", but, of course, the context was different.