I felt like the mage items cost changes eventually led to all these 'supports' happen

And I mean .

The cost reduction in warding+support item eg all lead to the mage support meta. Because to function well, they need items and back when the items would cost over 3k, this was near impossible to do as a support. However, now it's more feasable. This along with the mage update (malz voidlings + zyra plants along with the anti-engage from brand) made traditional engage supports like that much harder and they can't land their skillshots as reliably due to plants/voidlings can block them and others can just simply burst you down before you could utilise the engage properly.

My suggestion: increase prices, increased ap on mage items, lower ratios. Damage should be the same while it takes more time to ramp up damage (due to lower ratios) which will help with the burst meta as meta as well. As well as increasing time it takes for support mages to get the items and so their usefulness falls compared to traditional supports.