I think any of those should be fine but whenever I might be concerned about bitterness, I start with a sweeter wood, like cherry. Time is really hard to pin down, but I'd say try a taste after a couple of hours. For experiment's sake, remove some after one, three, and five hours to compare? If you're cold-smoking, good to go, but if you're hot-smoking, remember that water boils at around 212F (a simple reminder, because someone somewhere undoubtedly spent a bunch of wood and time, only to come back to... an empty tray).
For clear ice, don't waste time and energy boiling the water (even distilled water), as there will still be enough dissolved elements afterwards to disrupt crystal formation. Clear ice generally comes from (preferably slow) directional freezing, giving the dissolved impurities a chance to push out ahead of the ice crystals, rather than getting trapped between them. Camper English's
Alcademics website is pretty good on explanation and reviewing both DIY and bought solutions, including how to better use those silicone molds.
I'm not sure, though, if there's overlap between dissolved, clarity-reducing impurities that we want to push out for clear ice, and dissolved, smoky-flavor "impurities" that we want to hold on to...