First, weapons can be delivered through Poland:
Reznikov (Ukrainian Defense Minister), in his video, tried to provide an answer. “You may deliver it to Poland,” he told potential donors. “From there, we will transport them across the land.”
Delivery via Poland leaves Putin with 2 possibilities: attack Polish soil. And therefore NATO. Or extend his invasion to cover the Polish border areas, which is a lot more than most analyst expect him to have targeted at the beginning because it forces Russia to occupy all of Ukraine, more or less indefinitely.
Second, yes, it's an escalation. No doubt.
But an escalation in what?
With regards to Ukraine, Putin has already invaded. He's not going to "double-plus invade" because of those weapons. They are also, very deliberately, defensive weapons in nature. Anti-tank missiles, surface to air missiles, etc... The kind of gear that allows infantry to engages armored vehicles and ground support aircraft.
A country can't invade another with what's being delivered to Ukraine.
There's chance those weapons, and the courageous self-defense they are used for by Ukraine, will prod the Russian army to abandon its relative restraint in attacking cities and civilian areas. There's already a sign of that happening in Kharkiv today. That's on Russia though, just on Russia. They decided to invade, they decide if they want to be even more ruthless. They'll probably win, in the short term, with the numbers and gear they have. But every extra bit of violence makes it less likely that they will ever be accepted by Ukrainians.
It would be irresponsible and immoral for the West to use Ukrainians as "proxies" to weaken Russia. But it really, really, doesn't seem to be case here. Ukrainians genuinely do not want to be part of Russia's "near abroad" again and are only asking for weapons to defend themselves. In 2014, the West decided to cool things down and let Putin have his way in Crimea and Donbass. It seemed like a reasonable, if weak and timidly convenient, calculation at the time. It hasn't turned out that way, strictly due to Putin. Even now, analysts struggle to articulate an actual Russian-positive endgame to this, even if they win. No one really understands why Putin gave the invasion order on the 24th.
With regards to risking Russia-NATO conflict? Yes, there is a real risk there.
On the other hand, there is a real risk that not pushing back on Putin's Russia will embolden it to perform some other stupidity in the not too distant future. And, as others have remarked, the Cold War did see numerous events which involved supplying nations or groups fighting against one of the big powers.
And, to be honest, the more contentious pressure point seems more throttling Russia's central bank access to its foreign currency holding.
Then, again, when even Switzerland doesn't like you, you know you're not the most popular guy around.
If Putin's dream is really to go back to the golden days of the Soviet Union, why the Warsaw Pact era map looks mighty uncomfortable doesn't it? Are these countries all part of the fellow Slav brotherhood so dear to Putin?
Also, looking beyond Russia, which, for all its bluster and ruthlessness, is pretty much a has-been Great Power, except for its nukes, the West is also playing to the audience of China, a potential future adversary with considerably more potential. Not taking risks in supporting Ukraine now will make it difficult to negotiate any kind of equitable agreement with China say 20 years down the line. They will expect weakness from us and will act accordingly.
At some point China will become stronger than the US. If Western countries do not want to be controlled by the likes of people who run concentration camps for Uyghurs, but instead want to have a stable world where countries have real independence, might be worth to show some spine now.