Pellets are such s tiny part of the diet (just one tablespoon a day) but overfeeding pellets and not filtering their drinking water is where they get most of their calcium from so it’s best to watch the quantity of pellets you feed and filter water to control their calcium intake. Choosing a low calcium pellet can help.
You need to look at the first ingredient on each packet. The first ingredient is the one which the feed is based on. You are looking for a pellet which is grass or hay based. Ideally, you would then have a feed which is lower in grain (wheatfeed coming lower down the list ie not a second of third ingredient) or completely grain free.
Burgess excel is grass based. It does contain some alfalfa/Lucerne but it isn’t a main ingredient.
Harrington’s are sunflower extract based, with wheatfeed a second ingredient and grass as a third ingredient.
Science selective, the normal one, is alfalfa meal based with grains as a second ingredient.
Science selective grain free is Timothy hay based and doesn’t contain any wheat.
I have always fed burgess to my rabbits and originally to my guinea pigs but recently decided to switch my piggies to science selective grain free (now that I can get hold of it regularly!j.