Why do I feel this will go even worse for Russia if they do go for full mobilisation?
First of all, calling reservists quickly becomes a huge drain on the economy. Not only are these people not working and paying taxes, now you also have to pay and feed them. Secondly reservists are largely untrained, less disciplined, harder to motivate (especially in an offensive war) and as a result they suffer higher casualty and desertion rates than the regular troops. That in turn saps morale even more, but it also creates an operational headache for command due to their untrustworthiness.
Sometimes it’s better to know that you have left a flank unprotected and make plans based on that, than believing that it is protected when in fact the battalion of reservists who mans it will flounder and desert the second they start coming under heavy fire.
The war will get bloodier and costlier if they commit reservists. But mostly for Russia. Ukraine is already all in.