Some observations on what yiou have posted:
1. 90% of 20,000 = 18,000, if the numbers add up, you must have, on average several of each stamp ever issued by Rhodesia and Zimbabwe.
2. There has not been a King Henry in England since 1547. There will be very few Henries depicted on Rhodesia and Zimbabwe stamps.
It might be a good idea to post some pictures of the older stamps. That may give us an idea of what you mean by Rhodesia, as there are several territories that had "Rhodesia" in the name. Not all became the country of Rhodesia, now Zimbabwe.
As for your second question, you might want to see if you can find a Stanley Gibbons Catalogue to start with. Probably, you would be looking for the "Part 1" commonwealth catalogue. Note that the "Commonwealth & Empire catalogue" lists stamps up to 1970, when Rhodesia (now Zimbabwe) became a country. So, those might not be in that one. You would need a more recent "Commonwealth" one.
And there is the Rhodesian Study Circle:
https://www.rhodesianstudycircle.org.uk/