I've had holds use public testing (Curiosity, The 'Neatherhood) and private testing (3 in 1, DRO, Beethro Budkin's Big Burrito Butt Blast), but note that those last two involved me making a public post in
this thread on Architecture for testers. Neither method has really resulted in getting more testers and in practice the quality of the feedback ends up being solely on how lucky you get in getting someone who's willing to spend the time to provide through feedback. One good tester can easily outshine the sum of everybody else who tests.
Given that DROD is a hobby most people simply haven't got the time. I myself am guilty of this, ideally I would play more holds in Architecture and provide feedback, but I don't, because I haven't got the time (but on the plus side I'm an HA now so I'll likely end up playing it before release anyway, but not with the aim of providing any feedback for the author). The problem with hoping for the wisdom of the crowd to come along and help make your hold better is that there usually is no crowd until the hold is published.
What testing tends to be very useful for in practice is catching the more obvious unintended solutions. You should generally have at bare minimum one other person play through your work before release to catch these because as the author you tend to become blind to alternative solutions because you're too focussed on the way the room is supposed to work. For this, using a private beta board is
significantly more useful because of the way it automatically uploads a room demo when someone who has C-Net active solves it, because many times I've spotted some use an unintended solution for part of the room and not report it because they weren't even aware that it was unintended. When using public beta testing, people don't always post demos.
If you're looking to make your holds better and the rooms more enjoyable, then based on my own experience, the best thing to do is to make a variety of stuff, release it, and see how people react, both in terms of the anonymous score ratings and the comments in the threads. If you want to get experimental, make your hold short so you can more easily and quickly publish it to get an idea for how people feel about it. If you really want to maximise feedback, then I recommend intentionally making something on the easy side, as I've seen that people are far more likely to play it and comment on it if they can actually manage to beat it. In general people only have limited time and if they can't get past a room or two after a certain short length of time, the feedback ends there. Rather than ask for hints, they will instead feel that their skill is not enough and they are not the right audience, and just not say anything.