I was in an FFPC superflex best ball dynasty league where I did a rebuild to either build a possible contender, or to have it be good enough to sell for full price. (I ended up selling it for nearly full price.)
Commit to the rebuild early in the offseason. I've had teams where I'm stuck between "not good enough to compete" and "too good to rebuild," and then I end up with mid-round picks that aren't very helpful. Rebuilding sucks, because you are resigning yourself to commit to losing for a season, but if you start with that mindset early, you will be better off in the long run.
Accumulate following-year draft picks. If you commit to the rebuild early, then that means that at this year's rookie draft, you're already looking ahead to next year. If you're on the clock with a pick that has multiple guys you like, make every attempt to trade down while picking up 2025 picks. Easier said than done, but the draft is when everyone in the league is engaged all at once, and you'll get the most responses.
Target the weaker teams, but also target the oldest teams. Those old teams are going to do everything they can to make one more run, and a lot of times they do that by selling off future picks, so try to trade for those. (Another reason to commit early to the rebuild, so that you can be the first to target picks of the weaker/older teams.)
Be very active in-season, especially regarding injured players. In 2022, during the season, I traded a healthy Chubb for Dobbins, who was slowly coming back from injury. A few weeks later, I then included a healthier Dobbins in a package deal that netted Ken Walker, who I believe was dinged up at the time. (Dobbins then underwent a cleanup surgery, which was lucky timing for me.) A few weeks later, I traded Walker straight up for an injured Jonathan Taylor. Then, at the trade deadline, I traded Garoppolo (this is superflex) and Antonio Gibson for Dobbins again. (By the way, these trades were made with two different teams -- one won the title in 2022, the other won the title last season, so they were savvy teams.) Last year, I made constant offers of older, productive players for picks. Most didn't pan out, but you really only need one or two to accept to make it worth it.
If this is superflex, trade for future second-rounders. Most teams are unwilling to give up future firsts. But, especially in superflex, the second round is where you can make serious strides in team-building, especially if you can accumulate a significant portion of the round. You can use them to move up, or to take advantage of rookie fever by trading them for established young players, or just use the picks.