Looks like the Arenado contract replaces 2019, so it's an 8-year deal and not an 8-year extension starting 2020. That takes him through his age-35 season. An equivalent extension for Rendon would be 6 years, starting in 2020.
I don't even know if it's worth quibbling about a million here or there, but I'd have no problem settling on 6/$180 or something like that. Something close, but slightly below Arenado. Machado also receiving $30 million a year through his age-35 season. It seems like a pretty set market, so just get it done. Hopefully Boras won't be an jerk and demand a longer contract (ending age) or more money than two players at the same position who are clearly seen as better players.