Asides from everything else you said - because some estate privs are indeed very powerful - what?
Admin is far and away the most valued point type unless you're playing in the middle of HRE and/or don't plan on doing a lot of conquest. You need it for several key techs to expand your economy, and you need it for every war of conquest unless you're leaning big into vassals, at which point you also want high ADM efficiency for rapid integrations. Usually, you keep the +1 mana privileges, but many of the Estate Privileges you can slowly phase out in favour of ideas towards the lategame. I never do World Conquests because I don't have the patience for it, but you're really underselling how rapidly you can grow with high Adm.Eff., even in a casual game. I don't need 30% Manpower from estates when I can get it from conquering all of China in two wars.
There is MORE than enough admin points in the game to get to 6k dev, you dont NEED to save them. Admin is only something to worry about for the first 50 years of the game, when you need to focus it from tech 7 for about 20 years, so that you finish economic in a timely manner. By the time you get to absolutism (unless yorue doing a wc) I stand by what I said admin is useless.
Remember that theres been a lot of monarchpoint bloat since the game started, you now get priveleges, pp, level 5 advisors, you get much better rulers due to disinheriting. All this means youre average mana is something like 5 per month in each category, higher than the cost of tech and expansion is designed to handle. Thats why dev cost is so strong. You have excess mana, so you want to leverage it as effectively as possible.
And since you dont admin dev because its crap, you end up with tonnes of excess admin which you can use to core with.
Now playing meta deving, may not be the most fun way to play, but it is the strongest way to play. So if you want max absolutism and to conquer super fast brrrrrr, fair enough. But to think max absolutism is "better" than all the priveleges because it saves admin is flat out wrong.