How best to model client cohorts (subscription + churn) on monthly basis for multiple cases

Hi all, I'm really stuck on this one thing for a side project and I can't figure it out

I have a model that's using SUMIF to compile the monthly projections into annuals
(flows like 2024,2025,2026  ----     Jan 2024, Feb 2024, March 2024 .... Dec 2026)

It's a physical product so there will be churn.

First Question: Am I right in thinking that there's no shortcut and each cohort has to be modelled out individually? 
Cohort 1 purchases in Jan 2024, Cohort 2 purchases in Feb 2024 etc.
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I have been thinking about it as:

1. Reorder flag (1-0) for each cohort [e.g.80 rows]
    1.1 How to make this flexible so that from the assumptions tab if I choose "reorder every 4 months" it automatically changes? So the first cohort from Jan 2024 purchases again in April. Some offset formula?? [UPDATE: I figured it out. =OFFSET(previous cell,,-(Assumption of Month-1)]

2. Churn for each cohort
    2.1. Start with 100 clients, at month 2 there's 99, at month 3 there's 98 etc [UPDATE: I figured it out. =MAX(ROUND(Previous cell*(1-(((1+Assumptions for Churn)^(1/12))-1)),2),0) // calculates churn on previous number which isnt the same cumulatively as the inputted churn % but it works]

This part is important because it flows into COGS and re-orders. If I cheat by just reducing the annual/monthly revenue by assumed churn, COGS stays high because it's not being fed the real number of orders. [UPDATE: For COGS, when calculating amount that has been sold, just did Flag * Number of customers after churn)

3. Revenue [again 80 rows]
    3.1 Function of Ending # of Cohort (2.1 above) & average price of product at time (simple index match) multiplied by the reorder flag [UPDATE: Now fixed]

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I have been stuck on this for 2 days so any help would be appreciated!! Hope I wrote this in a coherent way

Final q: it seems like this is really repetitive and time consuming - is there a way to get 80% accurate answer with 20% effort? 

END UPDATE: I fixed all of it but still curious to hear if there's a shortcut apart from creating 3 'schedules/matrices/whatever it's called'