It's not an easy task to do in a spreadsheet.
But one simple solution could be to make a new column that makes the tags filterable, using an expression like
=CONCATENATE("|", REGEXREPLACE(B2; " *, *", "|"), "|")
which you then copy all the way down.
Then you get this (please excuse the semicolons in the formula; it's due to my Danish Google profile):
Then enable filtering by selecting Data | Filter from the menu.
If you then want to filter for tag b, you can filter the column "Filterable tags:" for |b|, i.e. with a vertical bar before and after the tag you want to filter for. You use "Filter by condition" and then "Text contains".
In this way, tags can even contain spaces, and you will always match the whole tag. (But of course it cannot contain commas, as you chose the comma as delimiter, and not vertical bars either, as I chose the vertical bar as delimiter in the "Filterable tags:" column. But those delimiters can be changed in the expression.)
In column B you can simply type the tags as you suggested, with spaces before and after the commas as you wish.
Here I filtered for |a| (sorry for the Danish text in the image; it's again due to my Danish Google profile, but I assume you can guess from the positions what the English text is):
Then I got this result:
It's not perfect, as every time you want to filter by tag you have to remember to add the vertical bars before and after. But it's at least one possible solution.
Note that searching for tags will also be possible by simply searching (Ctrl+F) for |tag|.
EDIT: Another expression might make the result prettier:
=IF(B2<>"", CONCATENATE("[", REGEXREPLACE(B2, " *, *", "] ["), "]"), "")
This results in
And then you filter or search for tags using simply [tag].