As promised here are the recent updates concerning my ongoing saga with trying to understand why my Asus router has slowed down of late. For those new to this a brief update first:
1. I have Spark Fibre Max - originally had 900mbps download speed - recently slowed to 250mbps
2. Troubleshooting with Customer Support concluded with the problem associated with my router.
3. Troubleshooting with Asus support who have been utterly ineffectual lead to no where.
4. Recently updated my router to the Merlin Firmware 380.68 and now getting speeds of 700+ mbps dl (I also did a pseed test just prior to updating the firmware and it was still 200+ mbps)
5. Have emailed Asus to say it is their firmware - I don't expect much in return.
Anyone facing similar issues may consider reading up about Merlin as an alternative firmware at https://asuswrt.lostrealm.ca/