B4’s so much more comfortable than A4 - you’re lucky!
In no particular order:
Yes, I think the default vertical spacing options are generally okay; it’s worth experimenting with one part and then rolling out those changes en masse.
I do make a point of working through part layouts from front to back, because the Frame Break function is just that - it doesn’t differentiate between left pages and right pages. If you go back and add an extra Frame Break early in the layout, it’s possible for all of your beautifully planned page turns to end up between a left and right rather than between a right and a left! So keep an eye on the page numbers. I tend to keep a spreadsheet that keeps track of the bar number of each page turn in each part, in order to ensure that 79 people aren’t all turning their page while a single bassoon is playing a solo (he said, giving away one too many secrets)
I have a blank master page (no music frame, but a text frame with “page intentionally left blank to facilitate page turns”) in my Parts Master Page Set. Generally this only gets used between flows, in situations where it’s too much of a compromise to stretch or cram the previous flow.
If you’re building a contents page or a cover page, either do that first - or at least put in placeholder custom master pages at the front of each part - or commit to doing that job in a word processor or other third-party application. If you insert pages at the beginning, later, you’re likely to lose any changes that are tied to individual pages: staff spacing’s the obvious one (and it’s rare you should need to tweak that manually, but even so).
If you’ve got a situation where you need to squeeze a few extra bars onto a page, it’s often quickest to use a Note Spacing Change (Engrave menu) that reduces the default space for a crotchet, followed by another Note Spacing Change that resets, at the top of the next page. This achieves the same result as the Make Into System/Frame functionality (which is a pair of Breaks where the first is set to wait for next) but you don’t have to think about where the spacing could be squeezed - Dorico can do the thinking for you.
The one situation where it’s not so great is if you have a tempo change (or other System Text) at the end of a system - the text will often hang over into the right margin. Either through time pressures or laziness, I tend to shunt that bar down to the next system.
If vertical spacing is lumpy, it’s often down to text that’s creating extra space above the staff. With the “set local properties locally/globally” switch set to “locally”, try overriding the Avoids Collisions property for those text items. You may have some manual Engrave mode dragging to do (again, with that switch set to Locally) but that’s generally easier than fiddling with the staff spacing tool.
Talking of that switch: I’m hoping you were working with it set to Globally when getting everything into the score. Either way, you should do a cross-check of at least one busy part layout - check that all the hairpins are hairpins and all the *cresc.*s are text, for instance.
If not, you’ll want to go to the score layout, in Write mode, select the first bar of the flow (top to bottom), then Ctrl-click the staves that appear in the vocal score (to exclude them). Then go Edit > Select to End of Flow. Then Edit > Propagate Properties. This will replace any inconsistent Write mode properties (in the parts) with the score’s properties.
Once you’re adjusting things manually in the parts - which hopefully you won’t need to do much of - you want that switch set to Locally or it’ll simultaneously be moving the same things behind the scenes in the score.
The other option you have for lumpy vertical spacing is that you can turn off collision avoidance between systems, within Layout Options > Vertical Spacing. I tend not to, but the option’s there if you need it.
Cues are best input in the score, where you can see what’s going on. Don’t overlook the “suggest” options in the cues section of the right panel. You can always lengthen or shorten cues from within the part layout. Oh, and cues can be copied and pasted (and duplicated to staff above/below), whether the actual cue or just the signpost. I tend to cue up everybody in one session before I start laying out the parts. And if there are particular conventions regarding what you want to see within cues, or which clef they should appear in, make those changes in Engraving Options rather than in the properties panel.
I think I’d better stop for brunch…