The "in-order" requirement makes this a weird problem to think you have. There's no situation where the channels are going to be heavier then the go-routines you're spawning to handle the processing: in fact the last line lamenting leaving blocked go-routines around is weird, because a blocked go-routine is still using less resources then one actually doing things - it's totally fine for them to block waiting to write to the channel because of the "in-order" requirement.

Your worst case scenario is you spawn N go-routines, and they complete 1 by 1 in reverse order from N so your entire dataset is waiting in memory for in-order completion - so no other concern here matters at all.