With "Advanced Strategic Icons" and similar mods, the icons are bigger for T3 mexes than T2 mexes, and T2 mexes have bigger icons than T1 mexes. This does make the level of each mex more clear. The higher-level mexes are more noticeable.
This doesn't exactly solve your problem, which is that you want to notice low-level mexes that ought to be upgraded.
UI Party not only has an accounting of your mexes (telling you how many you have of t1, t1 upgrading, t2, etc.) but you can click on those icons to "select all." So you can click on the t1 mex icon to select all t1 mexes on the map. If you're zoomed out and you do that, you should quickly see where the "safest" ones are that you can upgrade.
There is also a keyboard shortcut for "Select nearest onscreen lowest tech idle mass extractor." That is also a way to find T1 mexes that you might want to upgrade.