Preparing cedar board & batten siding for new solid color acrylic stain ... what makes sense for time
Well, you're in for a ride. Solid stain's application is really no different than paint's. For woods like Cedar and Redwood that bleed a lot of tannin, you'll need 2 coats of primer after sanding down to your desired look.

Meaning, if the Cedar has heavy grain like the first picture, then you only want to lightly sand. If you want smooth, then you'll sand a lot. The best results will be from Rustoleum's Deck Start primer, it's the only acrylic one and will therefore compliment your stain's structural properties.

As far as stripping goes. For any stubborn spots I'd go with Dumond's Smart Strip, it hasn't failed me like all others have. It's expensive, but won't harm you, the grass nor the wood. It can be a come-back-tomorrow-stripper, but I think your 1-coat will be ready in a few hours. So, continue your current methods and then Smart Strip the remainder.