Settings that reduce Outlook 2016 data usage on a metered connection, while allowing mail

Question

My (windows 8) machine mostly connects using a metered connection, to one Exchange email account and one IMAP account. I'm using the Outlook 2016 Trial, which has a nice feature that warns me before it connects when it sees I'm on a connection I've set as "metered":

My problem is that this is all-or-nothing: I have to hit "connect anyway" almost all the time because I frequently go for days between being able to use an unmetered connection, and then it just downloads everything.

What settings can I apply that minimise data usage while allowing mail to be sent and received whenever I'm connected?

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Here are a few things I've tried:

• Changing Send / Recieve ribbon > Download preferences to Download Headers seems like a good place to start, so that message bodies and attachments are only downloaded when I choose to open an email like in webmail (there's an explanation of what the options mean here), but I'm not sure if this might mean emails aren't kept after I've downloaded them, which could increase data usage if I refer back to an email chain with attachements.
• I got very confused about whether Cached Exchange Mode would cause data consumption to increase or decrease. I tried setting this to "on" while throttling it to the last week, thinking this would stop Outlook from bulk downloading hundreds of old emails unless I went searching for them (like how the Gmail android app downloads recent emails only), but it downloaded all my old emails anyway. All of the Microsoft guidance around Cached mode is written assuming unmetered connections that might be fast or slow - my issue is the amount of data used, not the speed.

I'm using Glasswire firewall to keep an eye on how much data Outlook is using, and currently it's using more than I think it needs - for example, 35mbs during a time period when I've only received around 10 emails, none with attachments larger than 500kbs.