Any lack of accuracy the missiles might have is due to issues with GLONASS itself rather than the missiles.
There were times a little over a decade ago when the satellite constellation was not fully operational. When you do not have the full constellation up the accuracy will go down severely. Right now the GLONASS constellation is fully operational and all the satellites come with more accurate clocks (GLONASS-M and GLONASS-K), and a new CDMA signal L3OC (GLONASS-K).
GLONASS accuracy right now should be around 5-7m or better.
Next generation GLONASS-K2 satellites are expected to improve accuracy by like an order of magnitude when or if they become operational.
Exagerated claims of lack of accuracy of Russian missiles like 30m or worse are BS. We have seen Iskander hit a Buk TEL which is roughly the same size as a tank, and even much older Kh-22 missiles hit small buildings. The Kh-22 was originally designed in Soviet times as a tactical nuclear weapon carrier to hit aircraft carrier battle groups. So clearly the accuracy is not as bad as that. The Russians also used cruise missiles to hit the flood gates of a dam in Ukraine a week or two ago without damaging the dam itself. If that is not accurate I do not know what is.
According to "The Washington Post", Iran is preparing a first shipment of Fateh-110 and Zolfaghar missiles.
This is basically a missile with similar characteristics to the Iskander. The latest variants do have more range though, up to 700km range, which means they should be able to hit Western Ukraine from Russia proper without using Belarus as the launch site.