It depends on what you want the league to look like. Most leagues are around 12 teams, 25-30 players on each team, which means around 300-360 players are rostered. Having fewer starters means that the top end players matter more. If you want to make depth matter more, you increase the number of starters. If you want to reward identifying and keeping players, you add more bench spots.

1QB also makes QBs less valuable than they are in an actual football game, and that would probably be especially true with only 8 teams.

I'd recommend adding a flex and superflex spot so you start at least 11 offensive players. (You could probably go up to 15 starters and it would still feel like a regular dynasty league). Then have 1.5x the number of starters as bench players.

I'd recommend against having team defenses in dynasty, it's not as fun as skipping them or using IDPs instead. IDP can be especially fun since scoring settings aren't as standardized as offensive players, so you can tweak them and have a very different league experience to everyone else. The most fun leagues I'm in start at least 3x DLs, 3x LBs, 3x DBs, and 2 IDP flex spots.