The 4 last players in our top 30 are only required to be on 72.5k next year. The rest of the squad has a minimum of 105k. You've named 18, so the remaining 12 would cost a minimum of 1.13 million, however there aren't many players in that group would be on more than the minimum. Maybe slightly above for Opacic, Mago and Pearson but not by much collectively.
Staggs will probably be on 105k next year as he is still contracted so we'll only bump him up to the minimum wage so we can play him without permission. Haas' contract will be backloaded to take advantage of future retirements so he'll be on less for next year. Fifita's contract was signed well before he debuted so probably less than 300k next year for him. Ofahengaue is probably on less until he signs his next contract. McGuire and Kahu are on more, Gillett and McCullough are probably on slightly more. Lodge also might be on a bit more, though I'm just guessing on that point, but he could certainly attract more than 250k elsewhere.
The cap is rising 600k over the next 4 years, so if we take that as a linear increase then we have a 2019 cap of 9.25M base plus 200k veteran players allowance and 100k car allowance for a total of 9.55M. If your total assumptions about those 18 players' contracts are accurate, with overestimations and underestimations canceling each other out, then we could probably offer Oates a contract of 500k. I don't know how close to the sun we want to fly though when it comes to the salary cap ceiling as I have no clue how much costs can blow out during the season.