The phased implementation is far enough along that "we are too small" is no longer a safe assumption.
What is actually required
Every transaction generates a structured invoice validated by LHDN in near real time, returning a unique identifier and a QR code. The invoice is not valid until validated.
The part that catches people
Self-billed e-invoices. If you pay a foreign supplier, or an individual who is not registered, you issue the invoice on their behalf. Businesses that assumed their accounting software handled it usually find this is the gap.