As Booking.com in our Perl ecosystem we have cases of runfiles reaching 70000 entries in total, making RBE really painful, and I've noticed sandboxing being one of the reasons.
Similar to Fuchsia we're working on a way tar our libraries (composed of multiple files) and our toolchain, and that makes the sandbox setup somehow manageable.
I don't remember the numbers but our setup time in RBE was in the order of 10 seconds our cleanup 7 and our test was another 10, when we started taring it setup/cleanup went down to < 2 secs.
I think the old rules for javascript would be a good candidate though.
Unfortunately I can't give you access to it, as in it's not available publicly, after BazelCon I can run with the flags you mention. If you're also going to Munich I would love to meet and talk about it.
Also we don't have a single 70k runfiles target, we have thousands of those, so the accumulate time is considerable. We tried with several options for local and sandbox_reuse was one giving us good results, mount in tmpfs also helped considerably.