Timing of longitudinal survey reminders

Hi everyone,
I have set up a series of longitudinal surveys that will be sent to the same participants over time: survey 1 (baseline), survey 2, and finally survey 3. Recruitment is staggered so participants will all receive the surveys at different days depending when they signed up, but all the surveys will be 3 months apart.
I have an Action set up whereby participants will be sent an invitation link to a subsequent survey in 3 months’ time. That is, survey 1 links to survey 2, and survey 2 links to survey 3. I would like to set up a reminder for those who do not complete the next survey, ideally 1 week after the invitation is sent. However, the Action does not let me customise the reminder timing in this way. 
I had originally thought that if I selected '7 days' for the reminder it would be sent 7 days after an initial invitation was sent. Unfortunately while testing, I found that the reminder was being sent 7 days after a participant completed the previous survey, rather than after the invitation.
As the Action only allows me to select a number from 1-30 and either 'days' or 'months', I can set up an invitation for 3 months but can't specify 3 months + 1 week for the reminder. I would ideally set this as 97 days or 13 weeks, but neither of those are options that I can select. Temporarily I have set it up so a reminder goes out within 24 hours but this is really not ideal - it doesn't give participants much of a chance to fill in the survey before they receive a reminder.
The person I was liaising with through Qualtrics support suggested posting in the community forums/seeking API support.
Also, not sure if this impacts on things, but I am already collecting data for survey 1 (baseline). I'm not sure whether I am able to apply changes to the Action while data is being collected.
Thanks for your help in advance!