Good points. It's difficult to find a clear answer to how important lend-lease was to the Soviet war effort. During the war, the USSR and US obviously had good things to say about the program, but the start of the Cold War soured this discussion, leading to the US overstating and the USSR understating the impact. Here's an excerpt from a paper by a British scholar exploring the topic. Emphasis is the author's:

It is neither possible nor fruitful to try and put a precise measure on the material value of allied aid to the Soviet war economy, if only because of the unavailability of many Soviet production data. Whatever the value of western aid, the Soviet war effort was measured in human life and suffering incomparable with material aid from outside. Further, the Soviet economy became much more of a war economy than other combatant nations. Nonetheless, it seems that the contribution made by deliveries from the USA and, to a lesser extent, Britain and Canada, played an important part at crucial times and in crucial areas. First, and above all, was a vital margin of food supplies, second was the provision of specialist or deficit products such as aluminium and copper, specialized tools, high quality steels. In this respect lend-lease supplies overcame bottlenecks. However, it must be stressed that the major impact came after the Soviet counterattack and the beginning of German retreat. Such aid directly and indirectly helped defeat the German forces, and was in such a way a substitute for a second front, but it did little to defend the USSR from the initial onslaught. Third, some of the raw materials and more especially machinery and transport equipment was of positive value to the Soviet economy after the war. For this, the tyre plant is the best but not the only example.

It is nonsense to repeat the figure of four per cent of Soviet wartime production and disingenuous to disparage western aid - a feature evident in Soviet literature and one criticized even by Khrushchev. It is nonetheless true (and this is a point repeated in some Soviet works) that Britain and the Empire received far more than the USSR from the United States. Lend-lease, in this respect, may be seen as a temporary substitute for foreign trade. Britain was a major trading nation, highly dependent on imports, especially for food and raw materials. The USSR, on the other hand, was an economy with little trade dependence whose foreign trade turnover had fallen steadily during the 1930s...

The part left off at the end compares repayment of aid sent to the British vs the Soviets. A fairly short read that will give some more context to the conclusions I shared above.

One of the main points the author makes is that lend-lease was used by the US as a stand-in for entering the war and opening a new front in 1942 as the allies (and Stalin in particular) were requesting. In this context, lend-lease was a replacement for reopening the Western front in 1942, an action that could have been far more impactful. The US provided material aid in lieu of entering the war, shifting the human burden of the war onto the other Allied forces and particularly the USSR from 1942 to least at 1944 (note that lend-lease aid extended wider and was provided from 1941-1945).

Overall, the impression I've gotten from sifting through academic writings on the subject is that while lend-lease certainly helped take some of the pressure off of the USSR (mainly in the form of producing food, trucks, and raw materials), it's most likely that the result would have been the same. That said, wondering over historical what-ifs, while fun, should really be constrained to recreational musing and shouldn't be taken seriously.