Well, it's up to you how you wish to conduct the campaign, but I will give a word of caution here: During those 5 years while you wait for that mine to finish, the Ptolemaioi will be spamming troops from several of their cities with their infinite bankroll. I made the mistake of over-roleplaying a Saby'n campaign once, only expanding outward in the 250's and 240's. I had to face an endless torrent of stacks, containing the aforementioned Kelrouchoi units, Galatian Heavy Infantry, Thorakitai, and eventually even stacks containing hetairoi. Having to grind down every single one of those stacks with nothing but skirmishers and the occasional axeman is still a source of trauma for me today.