Coriander is fine to be fed daily - a few sprigs. I don’t think there are many of us on here who don’t feed daily coriander. It is not considered high in calcium so although it is higher per 100g than lettuce for example, it is not a problem.
If you only feed lettuce, cucumber and bell pepper then they would be missing out on important trace elements in the coriander. Herbs are good for piggies for those elements but other herbs such as parsley and dill are considerably higher in calcium so are not recommended for daily feeding.
It’s all about a good balance and the items listed in the diet guide provide that.
Don’t forget that they still need calcium and if you go too far in trying to reduce calcium and only give them veg considered low calcium then you can cause just as many health problems as giving too much.
Pellets and unfiltered drinking water are the biggest contributors of calcium into the diet (even low calcium pellets contain more calcium than the highest calcium veg). If you don’t overdo pellets then you give yourself room to feed other veg items for variety.
I see your piggy has IC. Wiebke has said in other posts that it is changes to the diet and introducing new veg which can cause flares in IC piggies. Piggies can be sensitive to certain veggies but it is an individual thing and if your piggy is fine on the diet as being fed, then there is no need to change it (doing so potentially causing an IC flare)
Long Term Balanced General And Special Needs Guinea Pig Diets