I don't see Malik moving out that much, you have 2 30 year old DTs, you're gonna want to limit their snaps, say 60%, and Hargrave 60% as well, with Rush/Ridgeway getting 20% or so. Also factor in injuries (what's the over/under on Cox or Malik playing 16 games?) and this makes sense - you'll have a pro bowl caliber pass rushing DT on the field on EVERY snap of EVERY game.
Given the emergence of the short passing game in today's NFL, having DTs who can bring pressure within 2 seconds (not sacks, just forcing the QB to move his feet and take his eyes off his receivers for a second) makes it much easier to cover receivers. Top ten OTs can neutralize even top pass rushers for 3 seconds or so (don't have to stop them, just ride them enough so the QB can step up in the pocket), but it's really hard to keep a top DT from getting enough penetration to disrupt the pocket.
As far as injuries go, of course you expect some, but you don't pay rotational players premium UFA rates. For all of Howie's faults, that's not one of them. His contracts are exemplary. I also think you're likely very low on Ridgeway. They love the guy and I could see 30% even on a healthy roster.
Cox played 79.96% of their Defensive Snaps in 2018 and 78.33% in 2019. If they want to dial him back, I'm entirely in favor of it, but I'll believe it when I see it. Technically he was at a significantly lower portion in 2017, but they rested him for a game and he was injured in another. If you throw those out, he's right back in the 80% range. Jackson was trending toward playing 60+% when he was hurt in the Washington game. I don't believe that they paid him as much as they did to play him on less than 50% of their Snaps, so he's going to have to play more outside than he has in the past unless you're telling me that they're going to give one of them 2017 Beau Allen Snaps (41%). I expect Hargrave to be too good to pull off the field 45% of the time. He was already playing ~63% of Snaps in Pittsburgh last year in a much stricter role.
Jackson has shown the ability to move outside a bit, but for all of the reasons you've laid out here and more, pressure from the edge is a different animal. He's not going to win with speed/bend to the outside of a quality OTs, so he's going to play 5T. Now we also have to determine if Graham is going to continue to slide inside sometimes. There are a ton of moving parts here. Blanket DE/DT labels aren't the way to go anymore and not all DTs will overlap in roles.
I heart you.
what’s the price tag going to come in at?
I would guess 4/50 with 30 Guaranteed, but the way teams value Safeties varies wildly. I could see it swinging hard.