I have a few parallel lines of work on this - at one end I have an FPGA implementation (mostly coded but needs a bit of work to finish the basics), a logic-analyser/sampler for comparing synthetic output with real 6847 (bare metal coded on a raspberry pi) and a much simpler, and hopefully cost-effective implementation based on a RP2040 chip.
The RP2040 version has been stuck in implementation hell for over a year while I concentrated on the main board hardware
The current board is designed to provide a very versatile set of outputs based around a raw analog RGB output (3 bits per channel) as it is easier to verify through an oscilloscope but the plan is to switch to digital DVI output once the core is working - this basically gives a video-only hdmi compatible output. Mixing audio in would be problematic but not impossible
It is also possible to generate a YUV signal in the same way and feed that back through the main board but I'm not sure of the benefits beyond keeping the consistency of the original design