Not really. They can precompute those and inject it in an MP4 file so long as the settings match and it's inserted right before an i-frame so that it doesn't corrupt b-frames. They already reencode everything with their preferred settings, so they only need to encode the ads for those same settings they already do. Just needs to be spliced seamlessly.
But YouTube uses DASH anyway, it's like HLS, the stream is served in individual small chunks so it's even easier because they just need to add chunks of ads where they can add mismatched video formats, for the same reason it's able to seamlessly adjust the quality without any audio glitches.
Re-encoding is one thing, but ads are more or less supposed to be dynamic based on user location and likely some other data to target them.
Offloading that to the client made a lot of sense but now they have to do this server-side, they have very smart people working on making this as efficient as possible using tricks you've mentioned and more but it is still more effort than before. All for something that will likely be circumvented eventually.
All of that targeting data lives on Google’s servers already. Your computer isn’t trying to figure out who you are and what you like each ad play, Google already knows who you are when your browser makes a request for a video. Everything you are talking about is already server-side.
Every bit of effort and resourcing they spend on this returns revenue directly. Which is more than they can probably say for a lot of things they do. And they’re smart enough to know that they can’t eliminate blocking, just make it harder and harder so that fewer and fewer people do it.