But then it doesn't make sense to talk about R0 of a specific delta variant at all because that R0 for delta will be different for different seasons, different

But then it doesn't make sense to talk about R0 of a specific delta variant at all because that R0 for delta will be different for different seasons, different countries, even different cities etc. Instead you could say current R0 in London is 8 (or whatever) and it is mostly delta variant.