A serial of UN249 indicates a CPU-GPM revision which has the best video quality of non-1CHIP revisions. I don’t know about issues with sound. The sound chip in the SNES Jr is the same as the original model APU and 1-CHIP revisions.
They sell an RGB board here on the site but I cannot recommend it because it outputs the wrong CSYNC voltage (non-TTL) on pin 3 and also it does not have S-video. You would need to replace some components on the mod board to make it output TTL CSYNC to make it compatible with the same cable that your original SNES uses.
Voultar’s RGB board has a TTL jumper and has S-video support. https://voultar.com/index.php?route=product/product&product_id=51
The gscartsw_lite is the successor to the gscartsw 3.4. Superg is making a new revision of the gscartsw_lite which should be available soon.