No VBR dual pass for H.265 hardware encoding, stay with H.264 ?

I hate to burst your bubble, but the normal H.264 preset does NOT produce Blu-ray compliant files at all. The exported files MUST then be re-encoded and re-compressed by the Blu-ray authoring program, which does degrade image quality to some degree.

Instead, use the H.264 Blu-ray presets for exporting to Blu-ray compliant files. Unfortunately, hardware encoding is not supported at all using the Blu-ray presets. Only software encoding is available.

And old-school SD footage should NEVER be encoded into H.264 (at least the "standard" presets, as opposed to the "H.264 Blu-ray presets) no matter the cost, if image quality is important. That's because all old-school analog SD footage is interlaced, while hardware-encoded H.264 only supports progressive scan properly. And when it comes time to author your SD H.264 footage onto Blu-ray, the authoring software will re-encode and re-compress that anyway.

In other words, you're completely stuck between a rock and a hard place, in this case. Either make separate exports, or accept the severe quality loss due to repeated re-encoding and re-compression.

And lastly, you cannot export into an interlaced video format using hardware encoding. Only progressive or de-interlaced exports may be hardware-encoded. And you might have already degraded the quality of your captures by using a cheapo capture device to begin with: It always de-interlaces interlaced content using the brute-force "drop every other field" or "blend every pair of adjacent fields" method even before it exports to a file.