Decisions for building

That decision depends on the units involved and what you intend to use them for. Sometimes less is more. Sometimes it is appropriate to invest.

Keep your top units. Save rare and powerful skills like Bold Fighter only for 5* +10 units. Wait until you are well on your way with the merge project to inherit them. The meta can change and individual units' optimal builds can change too. You will regret it if you kill a top tier unit to inherit a skill that shows up for free in a seal or a refine just a few months later.

Generally speaking (since you didn't give any specifics), if the "Bold Fighter" unit is not one you would use very often versus a character you plan to use a lot I would say build the character you want to play with.

Fury 3, desperation 3, and bold fighter are all somewhat difficult to pull. Fury and desperation are in the 3*/4* pool, but so are 100+ other units (literally), whereas bold fighter usually has a 0.75% to 1% appearance rate but only on the right banner.

Use all of those skills judiciously, on units you will actually use, the more often the better, because lots of builds want them. Conventional or unorthodox, rare or budget, doesn't matter - what matters is that the build works well.