If I have an LtAP using LTE, is there a way to have RouterOS meter that data usage for each SIM card, over a certain period of time (as in, when the cellular provider's billing cycle runs) and preferrably send reports via email/SMS/webhook/curl?

You can do it by creating a script in your router to account for all LTE traffic inbound/outbound for the given period.
However, differently than for ethernet interfaces, LTE does reset all counters every time the session is restarted due to connection lost and re-established.
Also you need to take care to save temp data permanently when router is rebooted before sampled period, otherwise your count will be wrong anyway.