I’m not a Blender user, but 2.5 to 3.5 minutes per frame at 1920x1080 seems not exactly fast. While I do have similar render times (and longer), that is, however, usually at a resolution of 3840x1688 (i.e. approx. 3x the resolution)—unless we are talking about YouTube Shorts—and a corresponding render time of 7.5 to 10.5 minutes a frame would be more than I’d be willing to up with for an animation that make me no money whatsoever.
That said, I found that small tweaks in render settings (or the scene itself) can sometimes have a huge impact on render times with the render engine I use, while making practically no difference in the visual quality of the output.
Perhaps people better familiar with Blender have suggestions to improve flickering while keeping or reducing render times. I’m not sure that Eevee is going to help; in fact, because of the flickering, I thought the render was done with Eevee. (BTW: What kind of hardware did you use for this?)
I use Cinema 4D to render my animations, but I don’t re-rig the characters imported from iClone in C4D. Any changes to the animation (unless extremely easy to fix manually in C4D), I make in iClone.
Sure, “advantageous” framing (e.g. no feet in the picture) may help, however, sometimes it does not work for the frame composition. So far, I have not had to deal with uneven terrain, but if I did, I’d probably export a proxy of the ground to iClone to fix foot and/or hand contact there.
It may also help to go through the animation at 1/4 or 1/2 speed to catch problems. When I do cloth simulations, intersecting body parts mess up the simulation (up to and including a freezing/crashing PC after hours of simulating), so I have to be fairly meticulous when checking the motions of the character in iClone.
So far, I have only done “test” animations (quite a number of cloth simulations, and fluid, fire and smoke simulations) usually 30 to 45 seconds in length, but if I did a longer piece (perhaps after a hardware upgrade in the not too distant future), I would try to make sure that the “story” is on point and that as many problems (in terms of story logic, character motivation (why do they do what they do), assets to be used/combined) as possible are caught at the planning stage before I end up re-doing or throwing away many hours’ worth of rendering.