Western Artillery is at the heart of Ukrainian resistance.
They need substantially more and most importantly, the shells to maximize effectiveness.
France

is producing 78 additional CAESAR howitzers this year, which will all be sent to Ukraine, paid for by Ukraine, France and Denmark. All will be delivered in 2024.
Germany

is producing 36 Panzerhaubitze 2000s and 36 RCH-155 for Ukraine. Only some units will be delivered in 2024, with the majority of units being delivered in 2025.
Gernany

, Denmark

and Norway

have ordered 16 additional Zuzana 2 howitzers for Ukraine, with orders to be from Slovakia

in 2024 and 2025.
Czech Republic

has ordered 9 DITA howitzers for Ukraine but they're not due to be delivered until 2025.
Ukraine

claims to be building 6 2S22 Bohdana howitzers each month, but I'm skeptical of the claim that they're capable of bringing 72 units online each year. If the actual number is half that (36), I would think that's more in line with reality.
The above is a good start, but reality it's not enough. The following would go a long way towards helping solve the problem...
100 additional M777 howitzers from the U.S. stockpile of approximately 900 units. USA

Australian

and Canadian

donated M777s have suffered about 50% attrition over the last 2 years. A donation of an extra 100 would fully restock Ukrainian inventories.
Meanwhile, USA

has roughly 350 M198 howitzers in storage. While older, they still compare favorably to Russia's D-30 howitzers, which they they still use significant quantities of. USA is paying to store the 155mm howitzers and they would serve better utility in Ukraine's hands.
There also also substantial quantities of M109 self-propelled howitzers of various variants across Europe and in the USA. The Americans have already sent 18 M109A6 variants. They can easily afford to send 36-54 more, with 1400 units on hand (including 850 in storage), with dozens of M109A7 variants on order. Meanwhile, Italy

, Britain

, Greece

and Belgium still have access to a number of M109's that could be sent to Ukraine.
Lastly, Poland

has ordered huge quantities of K9 Thunder units from South Korea

. They've also ordered further domestically produced Krab units, however, they've also produced 72 Krab units for Ukraine. It would be smart for them to reserve at least 50% of their Krab production for Ukraine for the foreseeable future. Selling to / donating to the Ukrainians 36'ish units pet year would help offset the attrition these workhorse units have faced.
In terms of rocket artillery, it's simple. Ukraine has substantial quantities of HIMARS and M270s to create huge problems for Russia, but it's all about ammunition supplies. USA has upped GMRLS production to 10,000 units in 2024 and will further up production to 14,000 units in 2025. They're also set to produce roughly 1000 units of GLSDB in 2024 , before quadrupling that number to 4000 units in 2025. Then there is the topic of ATAM / ATACM missiles that are quickly being replaced by PrSM missiles. Donating a couple hundred ATAM / ATACMs to Ukraine would substantially improve Ukraine's long range strike capability in it's attempt to threaten strategic Russian assets.
Finally, Western artillery has to be supplemented by Western cruise missiles that can strike beyond the range of artillery. Britain

and France

have already provided SCALP and Stormshadow and both have been used effectively, in limited quantities. With the deployment of F-16s comes the availability of the American made JASSM cruise missile, which the Americans have thousands of in their inventories (they build about 500 units per year and are expanding production). Several hundred JASSMS would create a lot of problems for the Russian air defense and it would make many of their assets in Crimea extremely vulnerable.