I take from GP that they try to make their branches small, and keep the cycle of development->review->merging small, so that the problem stacked PRs seeks to solve doesn't materialize in the first place.

Stacked PRs in my experience has primarily been a request to merge in a particular order. If you're the only merger, as in GP's case, there's no need to request this of yourself.

Whenever I send a big diff. I spend some time annotating with comment first to helps the reviewer. A good summary of the changes in the description, the I annotate the diff of the PR, explaining approaches, the design of a specific changes, tricky part of the code, boilerplate,... Trying to guess the context is where the review bottleneck is, so I present it alongside the code.