Unity now has SVG support. I tested this and it's better (depending on the number of vertices) than having individual textures. Properly imported SVG images have no textures in them which prevents increasing of SetPassCalls and overall texture loading/unloading.
Overdraw is the problem for low-end devices and the best practice is to minimize the number of layers. However, something additional can be done.
First of all, the empty scene rendering should cost as little as possible. That means no post-processing, no unoptimized scripts, etc. Simple FPS counter and temperature measures of a low-end device will help here.
Then, the choice of shaders matters the most. The simplest forms of shaders are Unlit/*. However, they aren't as flexible as default shaders (which also can be considered as lightweight).
我来说两句