So I would propose, to also offer a limited amount of slots per stage (Alpha/Beta/BigNewPatch) to normal players to get feedback how "beginners" experience the game. Of course, you can make the selection completely random, so only the "big" backers are guaranteed to get alpha access, and to all the others its just luck. That way, you can keep the early access as a "reward" if thats important for you but still profit from the beginners feedback.
Proper alpha like for ED really need very selected group of willing testers, as they actually won't test finished game, but parts of it while putting it together. I personally know that I neither have proper box to run alpha (it will require more kick than finished game, due of all debugging infrastructure/hooks), nor I really will have time to run all these tests properly. All I want is to play almost finished game, that's why I got myself 80 pounds tier with lifetime pass for expansions, which also has second level beta. Heck, you can get 2nd beta even for 50 pounds.
Said that, during gamma period, which probably will be planned KS release date March, *everyone* who has pledged for copy of the game will have access to it. Look at it as public beta other games are doing.