I'd be careful not to jump from one extreme to the other. The Ex-Light has 1.8oz of down, while Montbell's Mirage has 5.3oz as mentioned. That's a huge difference. If you're going to err, it's nice to be the on the side of too warm but there are downsides as well with opting for an excessively warm jacket (bulkier, heavier, might be too much to sleep wearing, unequal insulation (way more insulation for the upper body vs. the lower)).
I recently owned an Ex-light vest and Montbell's Alpine Light Down Parka (similar to the Mirage but less down (4.3oz) and heavier fabrics). With that setup I felt like the vest was good for the warmest month of the year, and the Down parka was good for the shoulder seasons, but I didn't have a good general 3-season jacket. The Mirage is even warmer than the Alpine Light Parka. It's going to feel a bit ridiculous having that along May-Sept in most circumstances.
I suggest looking at something with 2.5-4oz of down. Keep in mind 2.5oz of down is already 50% warmer than your Ex-light, while 3.5oz is about twice as warm. That's a pretty nice way to go. There are a number of nice 3 to 3.5oz down jackets with a total weight around 9-10oz, such as the Western Mountaineering Flash, Montbell Guide Parka, Arcteryx Cerium LT and Patagonia Ultralight Down Hoody - the latter two of which are even lighter if you go no hood (not recommended).
FWIW, the Patagonia Ultralight Down Hoody can be had 40% off right now (see Gear Deals section) or the Arcteryx is $100 off here: