I'll leave it to Petard to answer some of the more technical issues. He is THE SME on all things artillery technology in this forum.
I'll say in general that modern artillery is significantly more complex than the 105mm C1 and the 122mm D30 of old. Once you get to self propelled guns there is also the issue of the automotive elements of the carriage that lead to complexity. When I was the BK of a Cold War M109 battery, we had 13 technicians (that's 10% of the battery's manning) to keep our six guns humming. Without significant ongoing, running maintenance, stuff stops working. When crews ride their guns hard and put them away wet, stuff stops operating. Not just breaking but frequently just gummed up. I doubt if Ukrainian batteries have anywhere near the detachment maintenance nor the tech support that our M109s had.
Barrel wear is a significant issue especially for these newer longer range guns and projectiles. You simply can't ram 95 lbs of steel through a barrel under enormous pressure without taking a little bit with it. The higher the propelling charge, the greater the speed and friction and the greater the wear. With the 105 mm C3 there are 7 propelling charges to the round. Firing Charge 7 (increments 1 to 7 combined) gave you the greatest range and thus the greatest wear. For every charge 7 fired the gun was considered to have fired one EFC (Effective [or Equivalent] Full Charge). If you only fired Charge 3 (ie increments 1,2 and 3) then you only fired a fraction of an EFC (I can't recall what the EFC values for each charge were anymore but let's say 0.25 as an example.). These vary with the different types of ammunition fired. So its not the number of actual rounds fired but the number of EFC's fired that determine the barrel life of a particular gun.
Other things effect that too. Around 2003/4 we introduced a new longer range round for the 105mm in Canada which used a new steel driving band on the projectile. This driving band was particulalry hard on the barrel of the LG1 and increased the wear on the barrel dramatically.
There are other issues too. On the M777 there is a loading tray for the projectile. The proper drill is to load the gun, fire it and place a new round on the tray after it has fired. Crews found that they could load faster if a new round was placed on the tray immediately after the first round was loaded and before the gun was fired. This non-authorized drill, however, put a strain on parts of the system and caused them to malfunction repeatedly. Similarly, the drill requires two men to manually ram each round. It can be done with one but this can cause improper seating of the round in the barrel. Detachment training and skill levels do matter - a lot.
Complex parts such as electronics and hydraulics can fail at the best of times but once put into field conditions they are susceptible to misuse or rough usage which increases their failure rates.
With respect to usage, that varies greatly. No gun in the Ukraine is putting 5,000 rds through per day. That may be the Ukraine's expenditure across the board but any given gun probably fires from zero to dozens to, at most on rare occasions, a few hundred rounds per day. Remember that on many parts of the front artillery engagements are very few and at best opportunistic when a target of value is found. During particular phases of an operation, such as near Kherson, that activity might increase for a few days while their supported arms are active. I don't want to compare Afghanistan to Ukraine but at their busiest, Canadian gunners with four to six guns were putting just over 3,000 rounds downrange during a six month roto. While some of those were at full charge at max range most were fired at much less than max range or charge.
I don't for a moment doubt that the Ukrainians are going to have maintenance issues. When we rushed the M777 into service two of the weaknesses in our preparation was the number of technicians we were able to train fully and the spare parts supply chain. Our wpns techs were learning on the job and for a while spare parts were coming off a damaged and cannibalized gun in KAF. We had issues supporting four guns of one model for a while. The Ukrainians have suddenly received highly complex guns of a number of models with very restricted parts support. They seem to be geniuses in keeping their older Soviet equipment on the road with local fabrication complexes, but there is only so far that even genius will take you when these types of systems are rushed into service.
As far as how many rounds do Canadian guns put through a year, I can't really answer for right now. Back in the 1970s we were firing off a lot of 105mm that was reaching shelf life expiry and our 12-gun L5 regiment fired approximately 10,000 rounds per year. That's roughly 850 rounds per gun per year. Virtually none of those were fired at Charge 7 (for much of that time we were limited to Charge 6 except in operational situations) so each gun was receiving substantially less than 500 EFCs per year. Barrel wear was not an issue at all and I've never hear of the L5s requiring barrel replacements before the gun was retired. The LG1s, on the other hand, in the mid 2000s fired far fewer EFCs but had serious barrel wear issues because of those projectile driving bands. Today's gunners fire far fewer rounds in training than we did back in the day.
Just an aside. You can adjust for barrel wear. As barrels wear down the projectiles velocity drops which effects the range to which it flies. If one constantly measures the projectiles' velocity then one can adjust the elevation that the gun is fired at to compensate for that drop. All of our guns these days have a muzzle velocity radar (also called a chronograph) mounted on the tube which provides accurate data to allow for those adjustments as the barrel wears.