Gonna take way more than that to get an elite defensmen in Sergachev..

It's the #10 overall pick in the draft on a team that can't possibly afford him. They may get a better package but I doubt they get a pick that high.

Add the 2nd from Isles (greene trade) and a prospect
As I see it it has to beat out a 1st/2nd/3rd of a OS from a team like Mont to even be considered

I'd throw in a 2nd from Greene but no prospects. Tampa has too many contracts as is.

Tampa easily declines, they are not trading Serg for anything barring a big overpay. The 10th pick and a late 2nd isn't even close. Tampa will re-sign him anyway around 4.5x3 so it's a moot point. However, if Tampa was going to move him to NJ, one of the few offers I've seen that even gets Tampa thinking is Arizona's 1st+Smith. Again, it won't matter bc Serg is basically untouchable and will likely be the first rfa bridged.

You might not have a choice. The cap flat is best case. You may need to sell the farm to be able to extend him and shed contracts. This may be your best option.

There is a flat cap, no best or worst case about it. There have also already been dozens(probably hundreds actually) of Tampa lineups for next year. Given Tampa's history with rfa's, the organization's structure, and the fact that the rfa's have shown no interest in leaving strongly indicates that none are planning on leaving. Depending on how much is left after signing Serg and Cirelli, it is possible that Cernak may have to be traded, but that likely hinges on if one of Johnson, Gourde or Palat waives. In any case, Sergachev is not the player they would move and certainly not for this offer. They will have plenty of options to fit under the cap before either "selling the farm" or having to trade Serg or Cirelli for 60 cents on the dollar.

I'd be pissed if I were him on that payday

Nah, because that's what he's worth on a bridge before n a flat cap league. The best two comparables are McAvoy and Werenski, who took 4.9 and 5 on 3 year bridges. Serg is no better than them yet and given the usual discount most take in Tampa, 4.5x3 is not unreasonable. It could be as high as 5 but doubtful. Had the cap risen as expected, Serg getting 5 on the bridge would have surprised no one. With the flat cap, 4.5-4.75 should be about right for Serg.

Just because there is a flat cap doesnt mean he will ask for less money. The market, aka the contracts he is offered, and the one he eventually signs will determine his value. The devils can offer him $6.3mm x whatever term he says and not bat an eye. Anything more than that, and they would need their 2021 2nd round pick, but if they acquire his rights through other means, a $7mm+ aav is not out of the question for any term length.

I see you've ignored all the comparables, which are often the best market indicators of what similar players are likely to get. Not many teams that he'd even think of going to are going to offer him much more than Tampa will anyway. The general consensus has been most players will be taking a little less on these contracts because of the flat cap. He's not worth 7+ yet anywhere, and doubtful at 6. Additionally, him signing with a cellar dweller in NJ vs staying on a top 5 cup contender in Tampa seems far fetched at best. Even if Sergachev were to sign an OS for 6 somewhere, Tampa matches without hesitation. Sergachev is the most important of the 3 rfa's and won't be leaving Tampa.