“mass shelling on a city of line can be effective but bad on ammunition consumption and barrel wear as well as counter battery fire.”
Russia inherited the old Soviet Artillery and Armor stockpile - the biggest in the world.
I listened to a podcast from West Point, where the guest American Artillery officer stated that Russia moved somewhere between 3-5 as many Artillery pieces into Ukraine, as the USA has in its current force structure (the USA has around 4,000 on duty, ~2,300 of them howitzers). So Russia has likely committed the great bulk of the Artillery systems that it has (10-18,000, rocket, missile and tube), and is losing or wearing them out much faster than replacement.
They have shot through a few thousand howitzer barrels (3-4?), firing about ten million rounds. Most of those guns could be made operable again back in the depot, with a new barrel, but they probably would have to newly make the barrels. The Ukrainian General Staff claims it destroyed or captured over 3,000 Artillery systems (likely including MLRS, as well as howitzers, maybe even mortars), but their claims are likely inflated.
The Russian shell stockpile is likely more depleted than its fleet of guns (well over half), but that would be relatively the least challenging thing for them to produce. Trained Artilleryman have also taken brutal losses, in percentage terms. Qualitatively, they have fired through or lost more of their newer and better guns, than the older ones, so that is a factor as well.
Overall though, despite historic losses, the Russians still have a lot of Artillery. It could possibly be among the last major combat systems they run out of, but they are well on their way toward that.
I would agree with everything you said and additionally they are replacing lost tubes with older less accurate kit. Results are 10s if not hundreds of rounds to do what one Excalibur can do. Firing that much leaves you vulnerable to sitting and counter fire.
Additionally I would think that large amount of old kit is not in that great of shape as well they had large sticks of ammunition what calibers abs what condition are questionable