You can run with very high OE - like 500% or more. There will be rebels everywhere, but no need to panic. Most of rebels are going to separate into a small nation that can be annexed in one war. So you can largely ignore those. If you strategically build big forts in defensible terrain in the areas where large nations might rebel they will have to spend a lot of time congregating and sieging, so you will have time to bring army and smash them. However, it's better not to come to the point when you have to do it, because it's a micromanagement pain.
My guess is that you aren't using vassals efficiently and that's why your pace is slow. Right from the start most of your ADM is reserved - you need to unlock 2 idea groups and get administrative ideas, so for the first 30-40 years your expansion should be almost excusively via vassals - besides they give you larger increase in your military power than if you held the land yourself. After annexing the first batch of vassals you get full cores and you can immediately decrease LA to get it down to 35%, so you will have a solid increase in your stable base. Influence + Admin ideas give you 45% discount for diplo-annex, so it's 4.4 DIP for a full core and 2.2 for territorial one. Vassals like to make a trade companies (when qualify) from the distant land of wrong religion/culture you give them, so you can use it to reduce your diplo-annexation costs (and time). Typical vassal play in Europe (where AE tends to be high) is to acquire a "seed vassal" - like taking a single cheap province with a core of largish nation or, ideally, diplovassalizing such small nation and then reconquering their cores (and in addition often taking an extra province for a new "seed vassal"). This generates little AE, doesn't cost much DIP to conquer and give you some extra military and tax from the vassal (and then you get their army and ships when annexing). They can also fabricate claims or have claims from missions (watch their mission trees) which can provide you with more DIP-free conquest.
More advanced play is to use ally feeding. Look for some OPM with militaristic ruler. Those usually want a lot of land (red) around them. Ally them, DoW the nation whose land they want bringing them in with a promise of land. Increase relations during the war and then feed them all they would take in peace deal. They will pay DIP and suffer all AE. Then you immediately diplo-vassalize them - because the LA of conquered land will be high they won't get large economic base modifier right away. You can use the favors you've earned to raise the trust. This is easier to do as monarchy than as republic, but I assume you're switching to monarchy at some point.
My guess is that you aren't using vassals efficiently and that's why your pace is slow. Right from the start most of your ADM is reserved - you need to unlock 2 idea groups and get administrative ideas, so for the first 30-40 years your expansion should be almost excusively via vassals - besides they give you larger increase in your military power than if you held the land yourself. After annexing the first batch of vassals you get full cores and you can immediately decrease LA to get it down to 35%, so you will have a solid increase in your stable base. Influence + Admin ideas give you 45% discount for diplo-annex, so it's 4.4 DIP for a full core and 2.2 for territorial one. Vassals like to make a trade companies (when qualify) from the distant land of wrong religion/culture you give them, so you can use it to reduce your diplo-annexation costs (and time). Typical vassal play in Europe (where AE tends to be high) is to acquire a "seed vassal" - like taking a single cheap province with a core of largish nation or, ideally, diplovassalizing such small nation and then reconquering their cores (and in addition often taking an extra province for a new "seed vassal"). This generates little AE, doesn't cost much DIP to conquer and give you some extra military and tax from the vassal (and then you get their army and ships when annexing). They can also fabricate claims or have claims from missions (watch their mission trees) which can provide you with more DIP-free conquest.
More advanced play is to use ally feeding. Look for some OPM with militaristic ruler. Those usually want a lot of land (red) around them. Ally them, DoW the nation whose land they want bringing them in with a promise of land. Increase relations during the war and then feed them all they would take in peace deal. They will pay DIP and suffer all AE. Then you immediately diplo-vassalize them - because the LA of conquered land will be high they won't get large economic base modifier right away. You can use the favors you've earned to raise the trust. This is easier to do as monarchy than as republic, but I assume you're switching to monarchy at some point.
