Flow to take Microsoft List multiple choice array, and return it to same list as a ; separated string (Or alternatively how to check if array contains a value)

[cfwk]

Hello! I've spent 2 days searching for an answer but cannot find anything I can use to help.
I have a Microsoft List (which I assume is the same as a SharePoint list, just abstracted?) with a Multiple choice column called Equipment which can contain (Laptop)(Monitor)(Mouse), then I have 6 more columns called Laptop Bought, Laptop Delivered etc. My intention is that if the Multiple Choice doesn't contain a value, e.g. Mouse, then the Mouse Bought and Mouse Delivered columns will display "Not Applicable" instead of "Not yet". I managed to Conditionally Format them to change colour based on this as that feature has IF CONTAINS or NOT CONTAINS, but I'm trying to do this with values.

The Calculated default value says I cannot refer to another column. So I thought if I use a Power Automate flow to take that array, and join it as a string in a hidden column, and then base the calculation off of that...

So far I have the following Flow: 

So when a new item is added to the list, the contents of the Array are selected, then joined into a string (compose was just to confirm it was working that far). Now my issue is how do I get that string into the List again? I tried to update item, but it asks be to re select every Compulsory field like Name, Start Date, Department, Manager etc. all I need is for one cell, the joined string, to be updated. 


Any ideas and I hope I was clear enough in my explanation! 

[cfwk]

That's the Update Item I see, and I only want to add to that yellow Concat of equipment field at the bottom, but I won't let me save without assigning a value to the fields, that on the list, are compulsory.

[cfwk]

What I mean by making the array a string is to then use an =IF(ISNUMBER(SEARCH([Concat of Equipment],"Laptop")),"Not yet","Not Applicable") sort of equation as a calculated field? As I cannot refer to another column in the default value calculation...

[cfwk]

And a further issue is =IF(ISNUMBER(SEARCH("Laptop",[Concat of Equipment])),"No","N/A") works in Excel, but is a syntax error in a calculated field making me think I'm totally wrong in how I am approaching this