You mute the track where a riff ends then generate the next track to play from there this will work the same way that BB generates a whole track.
If track 1 plays good until bar 19 you F5 mute @ 19 and freeze it
now you can leave track 1 soloed (and the Audio track) now solo track 2 as well,
track 2 will be F5 mute @ 1 so now we F5 back to normal @ 19
hit play generate until we get it to flow and fit from track 1 bar 18 to track 2 bar 19 and on until a good section of playing ends and a bad bit starts, then mute, freeze, track 3 F5 back to normal... and so on.
Track 1 maybe give you good playing in the first and last chorus but the middle don't fit so you just generate the middle on track 2.

Biab Track Injector for Reaper
Free Biab Chord Picker Tool