This is a genuine issue that is being worked on, but takes time to rectify...
On the M777 front, BAE has restarted a parts line for the M777, with barrels being at the top of the list. They signed an initial $50 million usd contract with the U.S. military for the production of parts and they will start delivering entirely new artillery units in 2025. When an order is announced, I'll post it.
In the meantime, there is another temporary solution being worked on and I think you'll see it announced soon as part of one of USA's Presidential Drawdown Authority packages. USA has more than 300 retired M198 howitzers in storage, with a robust compliment of spare parts. These howitzers are no longer in production, however, they're valued much less than the superior M777, but fire the same 155mm shells in that 30-40km range, depending on barrel and ammunition type. While older, they're still arguably more effective than the D-30 howitzers that Russia and Ukraine both use significant quantities of. Most importantly, USA can provide hundreds of thousands of 155mm HE rounds for the M198, where as it is limited in its procurement of 152mm rounds for the D-30.
The M1A1 Abrams tank is not a good fit for the Ukrainian military right now and never really was. It's a complicated tank, that requires a massive logistical chain. Furthermore, it's far too expensive to simply lose in mass quantities to cheap drones. This isn't a problem unique to Ukraine. It's also why Russia has pulled most T-90s back from the front and assaults using older, cheaper, T-80s, T-72s and T-62s instead. They were getting tired of losing their multi-million dollar tanks to an onslaught of dirt cheap FPV drones and land mines.
USA never wanted to to send the M1A1 Abrams. It doesn't make sense for Ukraine because the American version uses depleted uranium armor that USA refuses to send to the Ukrainians. They simply sent 31 stripped down versions because Germany would not send Leopard tanks unless the USA was on board with sending their tanks as well. The best use of M1A1 Abrams moving forward would be sending them to Poland, in exchange for them sending more of their Leopard 2s to Ukraine. Poland is moving to a fleet of Abrams and K2's anyways, so this makes sense for both Poland and Ukraine if the USA has the political will power to make it happen.
In a world where FVP drones and Lancet drones are taking out Abrams and T-90s virtually just as easy as they are T-64s and T-80s, both sides are using their older inventories and saving the majority of their best weaponry from being destroyed so cheaply and effectively. That's why there is a huge push to send 200'ish Leopard 1A5 MBTs and whatever is left of T-72 MBTs in Europe, as well as to refurbish T-72s and T-64s from Ukrainian storage. Given their respective roles, you get virtually the same capability, with a similar chance of survival, at a much lesser cost.
It's also why USA just announced that they are sending another 1000 multi-variant HMMWVS and 300 additional up-amored MRAPs to Ukraine. Speed and maneuverability has become more important than anything on a battlefield infested by drones. Second to that is survivability of the crew, which modern MRAPs specialize in.