My rule of thumb is to have enough coins, supplies, goods, and FPs in inventory to research the next age's entire tech tree, negotiate any undefeated provinces of that age, negotiate all of GE for a month, and rebuild my entire city to the next age's buildings - twice.
Since I take advantage of RQs, I want to be able to run through the Tech Tree, Story and Bonus Quests as quickly as possible to get to the next age's RQs and RQ buildings. Since each age has 3 or more RQs tied to specific production buildings, I want to unlock them quickly, and completely rebuild my entire city with each new building as I unlock it and, if nothing else, mine those RQs for their diamonds. If you need residential, it also means the new age's highest density housing, which is almost always at the end of that age's tech tree.
So for me, going from HMA to LMA, means unlocking and building Cooperages as quick as I can while continuing to sprint to the end of the Tech Tree for Breweries and Apartments. Once Breweries are unlocked, I replace half my Coops with Brews to mine the Coop, Brew combo RQ for diamonds, then replace the rest of the Coops with Brews once the diamonds are in. If I've still got residential, I replace Townhouses with Apartments as quickly as I can make the transition. When I do my complete rebuilds, I buy the 48 hour tavern boost which shaves 50% off the time. I've found the 72 hour boost which shaves 75%, builds so fast, it lasts days longer than the rebuild takes.
HMA troops will easily get through GE 1 and half of GE 2. Beyond that, for the first month, I negotiate the rest of GE to build up a the new age's supply of unattached units of troops.
I have no barracks in my city, so my goal is to not to build any barracks in the new age and to leave any military tech I can unresearched to use for Events later. I also don't wan't to lose any of the previous era's now irreplaceable (without Traz) unattached units while building up the new era's units to fight further into GE and eventually the C-map.
I want to get to the end of the LMA Story Quests quickly also to open the second quest giver slot. Then I'll tackle the Bonus territories so once I'm done I can get rid of the check mark from the uncollected 'Fair Maiden' quest.
So for me, it's about having specific goals to reach in the next age, then stocking up everything I need to get there as quickly as I can, immediately rebuild, and get right back to spinning max RQs to stock up on the goods of the new age and everything I'll need for the next sprint into the next age.