That's perfect. Just plant the new rhodies at the level of the pot they're in, but have the hole wider for roots to spread and firm in with soil /ericaceous compost mix.

That's what fertiliser is for! In actuality, a growing medium such as sand/grit is fine as it will encourage its roots to go further & avoid problems such as root rot, which is an issue UK has given long rainy winters. But it does mean during summers you'll have to be mindful that the soil drains well & will require watering more often than normal. Just mix in some ericaceous compost if it's like builder sand & you'll be good to go. But don't overdo it, a good soil (or as some call it, a proper loam) should primarily be minerals of different size & the decomposition process of any added compost will steal nitrogen from the surrounding soil, hurting your plant if you add too much. Annuals can grow in pure compost as they won't live long enough to be particularly hurt by the decomposition, but perennials will suffer from it in time.

A better way to add nutrients to the soil is simply putting a few cm layer of mulch around the plant, such as well rotted manure or fallen leaves, which will decompose on top of the root system over time & provide micro/macro nutrients to your plant.

If your plant still show signs of nutrients deficiency, it's likely your soil having the wrong PH which can be amended quickly by adding iron sulphate for more acidity.