I have Ts2020, instead of buying all the deluxe add-ons can I buy Ts2021 deluxe as a separate game? Thanks.
Yes you can just buy the game and it will add all the routes as if you paid for them separately. Your game will updated to ts2021 anyway, you just won't get the routes unless you "re-buy" the game.
I'll just add that at the point of purchase you will get a warning to the effect that you already own some of what you are buying and won't get another copy. This is because you already have the core program- acknowledge the warning and you can go ahead. It's also worth mentioning that Steam keys for older editions of TS are still available on other sites (prices and availability change often so I won't post a link but you know how to use Google) and you can use them to add routes too.