Some destruction has been lacking, it's definitely a key aspect of robot combat, but as ever you are on the ball Jonno with what the events need to keep them exciting.
I think it's sensible to limit the main factors. As you've said, it saves you from having to completely overhaul your arena and it allows a degree of destruction without posing as large a risk to the public as full-on spinners would.
Marien's suggestion of restricting verticals could be a good decision too, as you're likely to get a lot less vertical flying shrapnel from horizontals and most of it will be hitting the polycarb instead of the netting.

Limit the construction and material of the disc
Maybe better to not limit this aspect? Dunno, just thinking that if the disc is made from as high a quality as possible, you're less likely to get, say, teeth shearing on it compared to a disc built from less suitable materials?

Quick question as well if this was to go ahead: what would the situation be with the FW spinners? Obviously they're a lot smaller and the chunks that could be torn off them are likely to be a lot smaller and lighter, but we all know the power some of the FW spinners can pack. Would they be allowed to fight freely or would there be restrictions on their motors/disc size/speed etc too?