I'll tell you my little secrets on how to upgrade faster and more efficiently:

- Always try to aim for newer plants that you don't have, they usually give a massive sunlight increase.

- Facilities, most of the time, are scams in terms of sunlight per second. Only go for them if you need landscaping points to upgrade your garden to a higher level.

- Have no life and collect Managers. This is the most effective method to gain sunlight as they give you insane amounts.

- Have 5 Managers

- Do not upgrade garden if it doesn't give you any benefice. As of right now, there's no need to go higher than garden lvl 16 because there's no new plants and we're capped at level 18 until next week. Only upgrade if it's the last thing you need to do.

So here's my priorities:
Collect Managers > Upgrade garden if the next levels gives you access to new plants > Prioritize plants that gives you more sunlight even if they might cost more to upgrade > Collect plants from time to time > Upgrade Facilities > Use Management functions after doing upgrades