So sorry your little one is poorly. DD1 was in NICU with GBS sepsis for two weeks and although she was our first, we saw lots of other parents on the unit juggling older children and NICU stays.
Their approach was very much like it sounds you are planning - there during the day, would go home again for older DC tea / bath / bed and often would tag team again in the evening presumably once the older DC were asleep. Its really hard, especially when you are recovering as well. The staff were very conscious of who had older children at home and spent lots of time / extra cuddles (when possible) with those babies.
Have they hospital loaned you a proper pump? They should also have an expressing room so you can express whilst at the hospital without having to bring all your pumping equipment with you each time.
You can ask if they have an online system - ours did this thing where they would upload a photo of the baby at certain points in the day for you to log in and see if you were away from the uni.
Have they given you muslin / fabric squares? If so, wear them in your bra each day and then put them in baby's incubator when you are not there. Baby will be able to smell you.
You may also find the stay isn't as long as you expect. Once DD1 was uprated from NICU to SCBU, they basically wanted her out ASAP. For us, feeding was an issue and delayed our discharge as it all affected my supply, she wasnt gaining etc. If you've successfully fed 4 other DC, you may not have that problem and providing baby gets sorted with latching, you might get home quicker than expected.
Please also take care of yourself. I did not and it held back my recovery. If your mum / sister cant help with childcare, could they drop off some food for you (like a big lasagne / cottage pie etc) to at least take the pressure off cooking? If not, i also recommend filling your freezer up with ready meals or from Cook Food so you can grab something when you get in from the hospital.
Hopefully someone with personal experience of older DC will be along. Good luck and hope baby is home soon!