The pattern is most likely coming from something on or just above the sensor stack, not from the lens. Based on the answers, the two main possibilities are:

1. Fungus on/under the sensor’s filter stack
2. Cracking or delamination in a thin film or the IR/low-pass filter layer

Both can create visible marks that become much more obvious at small apertures, which fits your tests. A normal surface dust issue usually looks different, and basic sensor cleaning would likely have changed it.

If it’s fungus, it may be cleanable, but usually only with disassembly. If it’s cracking/delamination, the affected glass layer may need replacement or removal. On an older body, repair may not be worth it unless you want to experiment with an IR conversion.

Practically, if you mostly shoot wide open, you may be able to keep using the camera since the defect is far less visible at wider apertures. To test sensor-level defects in the future, photograph a plain bright surface (like a clear sky or blank wall) at a narrow aperture.