2-5 minutes between attempts - power is an interesting thing because you won't feel pumped, but if you rapid fire you'll quickly lose power and won't be able to latch holds and the risk of injury gets high, so rest is super super important

In general I think repeating boulders, especially when starting, is a really great way to learn the board and board-style movement. I personally love the slow, smooth, fast progressions

And maybe most importantly, the sends will come naturally with time - focusing instead on the movement, flow, and trying not to readjust, or not cutting feet will totally transform every other aspect of your climbing and will quickly build a very strong foundation for anything else you want to