Is limiting communication with relatives to holidays and special occasions, and merely greeting them upon meeting, considered a severance of kinship ties?

"Womb" (Al-Rahim) refers to relatives. Maintaining ties with them is according to what is customarily accepted. So, whatever people recognize as maintaining ties (silah) is indeed silah, and whatever people recognize as severing ties (qati'ah) is indeed qati'ah. If the communication you maintain with your relatives is sufficient for upholding kinship ties according to the custom of your country, then this is sufficient, and you will not be considered negligent.