Now I love a guy with a plan. I hope it works for you. I do. I would observe imo the following
The ruck line is the hardest line to fix if it goes pear shaped
To go Witts/Reeves you need a certain fallback position if something bad happens. I do not think Preuss is it but kudos to you if you think he is the guy. The chances of picking up a mid priced ruck at short notice that scores okay because you copped a ruck injury (eg Witts) are questionable.
I am not sure of your midfield but imo you need at least 3 clear go to captain options each week. Nearly always one of those options is the R1 because top rucks constantly score big and having the C on them gives a big advantage. You could not rely on any of those rucks to take the C. Are you starting say at least Macrae, Steele and Oliver/Bont?
The guys you mention you are bringing in all come with risk. Probably not so much Cogs. He is much more like the Zeibell/Hall scenario of last year. Known players that for some reason are cheap but can go big and/or have a role change. Rioli has never gone big (one ton in 3 seasons) and it is unknown if the role change will produce the SC scores we would all want. Likewise Coleman.
While you do not want to start too many rookies, they are your easiest cash generation-provided you get the right ones. So you need enough of them to make money. You have Coleman pegged at around 80. He needs to be around 86/87 to make $150k. A $117 rookie only needs to ave around 58/60 to do that. Plus the $150k Coleman makes still only gets you to just over $400k so you still need to find at least $100-150k to get a top defender. But Coleman probably has better job security than many rookies.
I have no desire to steal the jam out of your donut and if what you propose came off it would be huge. But from past experience I think it will be very hard to get that many speculative picks right. Good luck.
This is what I like about this site, people willing to give there time and effort with an unbiased slant in response to questions/statements, so thank you.
I'm not trying to convince anyone about my take on things, it's more about establishing in my own mind that my strategy might have some merit, so to just counterpoint some of your points.
A fallback position for me if Witts goes down ( a big man coming back off an ACL certainly has risk attached) then I can drop say a Sicily type down to a performing rookie and get one of the big dogs as R1 (I'm happy to use Preuss in this role if he's rolling ok) so 2 trades for the Sicily solution as against 1 for Preuss. Of course I don't have to use Sicily if he's performing well, there will probably be an underperforming premo in my team anyway.
I have Mitchell, Walsh, Petracca, Jelly and Neale in the mids with Lloyd back and Dunkley forward. There's certainly a cost in not having that R1 as a C option for those rounds until I can upgrade.
Coleman is an interesting one and his position in my team is completely reliant on him maintaining the role that he had for the last 3 rounds of the year where he averaged 85, I'd be stoked if he kept that up because, as you said, I'd only need a maxxed out rookie to trade both to a top defender, whereas I'd need 3 rookies at least to achieve the same outcome. It's exactly the same line of thought that I'm using for Rioli, in 5 of his last 6 games he averaged 91 (excluding a 54 because of 7 clangers). The combined benefit of those 2 players over rookies is possibly 65 points/round for around the same cash generation as well as using 2-3 less trades in turning them into premo defenders, the other advantage that those 2 provide is the DPP flexibility in tandem with De Koning, Skinner and Kelly. Cogs to me is a no brainer.
The combination of Cogs, Rioli and Coleman replacing rookies could be around 100 pts/rnd somewhat compensating for my weak ruckline, but come upgrade time (and with a bit of luck) I could add a premo/week using just 2 trades/week for 4 weeks hopefully putting me ahead of the pack and only needing 5 more upgrades to complete my team.
I'm relying on 5 speculative picks to achieve averages that they have either proven to have done in the past or should do given the roles that they have been pegged for. I actually feel that I'm taking more risk with players like Sicily, Maynard, Heeney and Thomas (who aren't set in stone as picks yet)