Can we abstain from billing the days/visits that we see post-ops in order to not disrupt the referring and comanaging billing for the full 90 days? Or are we required to bill for the days that the patient is under our care? 

You should report and bill for the post-operative care that the surgeon provides, and the comanaging provider should only report and bill for the post-operative days and time they provide care using modifiers -54 and -55 for those payers who recognize these co-management modifiers.  

From our 2022 Fundamentals of Ophthalmic Coding (aao.org): 
“The amount paid by Medicare Part B to two physicians is never more than the total global surgical fee provided by only one physician. The surgeon must keep a copy of the written transfer agreement in the patient’s medical record. The resource also contains an example of a Comanagement Model Consent Form. The physician assuming care must provide at least one service before billing any part of the postoperative care. Most practices do not submit a claim until the end of the global period.”
For ethical questions about comanagement, refer to Fundamental Ethical Issues in Delegation and Comanagement - American Academy of Ophthalmology (aao.org) and Comprehensive Guidelines for Co-Management of Ophthalmic Postoperative Care