My opinion:
Canon's naming "conventions" can be given some rationalization in hindsight, but are actually unpredictable. For example, no one outside Canon would predict splitting the 5D series into two after 5DII, nor would expect Canon to come up with RP (and then not with RS, but with R5) either.
Actually, the proverbial "5-series" started with EOS 10 (EOS S in the U.S.) in 1991. Then it became EOS 5, then EOS 3.
EOS 5, released in 1992, already had the eye control AF.
"6D" series was a lower-tier spin-off of the "5-series" after the 5DII. Only two models of the 6D series have been made.
The closest positioning of the R6 would actually be a continuation of "pre-5DIII" 5D series. Similar price to 5DII, similar ergonomics, similar build quality.
"7D series" has a history of having only two models and being the slowest updating Canon series ever. EOS 7D was released in 2009, EOS 7DII was released in 2014.
Actually, the closest analogy to EOS 3 in Canon's digital lineup would be EOS 5DIII. EOS R3 is a completely different beast.
Canon's Rebel models traditionally have 3-digit and 4-digit model numbers.