I run my shop on Shopify which has Google Shopping integration that I took advantage of. Many of my products are either custom-made or manufactured by small crafts people who don't bother to get UPC/GTIN/MPM codes for them. I set the "custom product" flag on those products in my Shopify configuration but noticed that in Google Merchant Center it still "complains" that they have limited performance due to the absence of a GTIN, MPM or brand. So first question: is that okay because limited performance does not mean no performance? It seems to me that setting that custom product flag should override the requirement to have those attributes, but it doesn't seem to.

Next question is that while my sales are fine and slowly growing, I do wonder if I'm missing some low-hanging fruit by not having my Google integration set up properly. I can't afford right now to hire someone to audit or manage it for me. Is there anything I can/should check on myself? I have used Google Analytics on occasion and get highlights but tend to be completely overwhelmed by the configuration/reporting options in it, not to mention the time required to learn it and use it well. So... low hanging fruit, anything simple that many businesses miss that I should check?

Edited to add: I decided I wanted to add a clarification of my thinking on this, particularly the first paragraph. The items that I don't have codes for (small, crafty manufacturers) tend not to be sold on Amazon or other major retailers, so if I can get them seen I think they'd sell better. But I think they're not being seen because of the missing numbers. Also, I have tried to search online for similar products so I can use their codes, but the search results seem to be way, way too specific. For example, searching for "window decals" comes up with very specific designs and manufacturers, which I don't think it would be ethical or accurate to use. Is there a way to find codes for something less specific, like "floral window decal"?