One of Casas' most similar batters through age 23 is Mark Teixeira, per B-R.com.
Fifteen years ago, Tex did get an 8 yr / $180mill ($22.5mill AAV) deal from the Yankees. By the time he got that deal, 29yo Teixeira was more established than Casas is, and had won multiple Gold Gloves and Silver Sluggers. More recently, Freddie Freeman, a clear future Hall of Famer, was only getting 6 year / $162mill contract ($27.7mill AAV). Freeman was 32 years old, but had a Gold Glove, multiple Silver Sluggers and an MVP under his belt.
These are among the largest contracts ever awards to first basemen. So yes, they really don't break the bank anymore. Casas has age on his side, but also has less hardware than Evan White (4 yr $24mill contract with Seattle, now considered a bust). I don't see Casas breaking the bank. A six year deal like the one above would buy out all his arb years plus first two of free agency and take him to age his age 30 season. He certainly could wait it out and make more money, but an early deal like this buys him plenty of security, but might be better if it was a year shorter and allowed him to reach free agency before his age 30 season. A 29yo free agent just has more earning power.
I would agree an extension like this, but maybe 5 years before the option probably makes more sense for both sides...