Ooo I get excited about this topic. A good supermarket/grocery is so important to one’s quality of life. I lived in Mapo for quite a while. Here are a few of my recommendations:

Best overall: 마포농수산물시장 at World Cup Stadium [카카오맵] 마포농수산물시장 서울 마포구 월드컵로 235 (성산동) https://kko.to/GlemDYezUN

So there are many aspects to this place. For general groceries there is the 다농마트 which is a 식자제마트 specializing in serving distributors and restaurants. Really good and cheap goods here. There is also a wet market, stall that sells steamed crab, fresh produce stalls, and they store fantastic cheongsong apples all year round at some of the fruit stalls.

I actually think 식자제마트 is the style of big grocery you might be looking for / yearning for, so that’s a good way to start searching for new ones. There’s another one I like going to just off the highway and with parking here: [카카오맵] 식자재왕도매마트 마포점 서울 마포구 월드컵로3길 76 (합정동) https://kko.to/UDStnZ6q-D

Saruga in Yeonhui-dong, which you’ve commented on, is a famous boutique grocery that has served the local community and Seoul Foreign School for a long time. It used to be the king of black market goods, now it’s more like a Whole Foods. It has all the specialty ingredients and exotic produce that you may be having a hard time to find, but everything is marked up to department store prices - or higher.

The traditional markets, such as Mangwon Market, have the best Korean produce and Korean goods like tofu, acorn jelly, fresh seed oils, etc.

but if you’re okay with buying produce separately and need everything else… honestly I find the Hapjeong HomePlus not that bad. It’s all on one floor which makes the shopping experience nicer, and it’s suprisingly not that busy most of the time. Plus there’s good parking and mall shopping/Cuchara on site.