Taxi - IMHO, I would say having Taxi slots helps out people. If someone is good at rookies or not, they will have better rookies than other teams regardless. Those Taxi slots allow people to potentially be wrong without having to cut players from their active roster. I personally do not see a downside to them, especially if they are locked once the season starts. At that point you only pull a player off the Taxi as opposed to adding them in-season. Also only allow Taxi for rookies. Whether they play or not, after their first year they are no longer Taxi legal. But Taxi slots are definitely not necessary and these are only my thoughts. Not right or wrong
One thing not addressed here is how to draft. IMHO, auction is the way to go. As a start up league, and especially dynasty where you intend on holding people indefinitely, I like the idea of everyone having a chance at any player. They might not win the bid, but they at least have the shot. This allows much more flexibility for roster building. Further, with a start-up there are no previous winners/losers. Giving someone a 1st choice in a forever league can be a huge leg-up due to simple randomness. Letting everyone have a chance to bid gives everyone a shot at any player effectively levels the field. Same for the rookies, or just roll them all into one massive draft which is fine at this point since the NFL draft has happened.
Auction will take more time, no doubt, and I have found ESPN a great platform to host an auction, most importantly it is free. Transferring over rosters takes a bit of time, but not really hard to do. But ESPN was super smooth and easy. You definitely do not need to do this, but I think it makes for a more fun and interesting start to the league.
All of this is only my thought, but hopefully it helps!