I don't doubt that the invasion of Malaya would be delayed until after the conquest of Indochina. Nor that Japan in OTL would use closer bases to support an invasion. Regardless this doesn't mean a Blitzkrieg is impossible anymore than Japan not acquiring Borneo prior to the war meant they couldn't Blitzkrieg OTL.Launching an invasion is one thing, maintaining it is another, and the Japanese in 1941 maintained their operation from Indochina.
If the supply dumbs are initially located in Hainan then it's a significantly longer journey but as we know from OTL Japan did not need to use their wartime supply routes for long because they conquered Malaya in 2 months and could move supply dumps to Thailand almost immediately after the war starts.
Particularly as this is before the Franco-Thai war in December 1940.
I don't think it is so easy to say that a war in July 1940 against only European powers (if Britain even intervenes) is worse for Japan than a war in Dec 1941 against EU + USA. There are downsides and upsides.
In addition paint a map of the world in the Axis in July 1940. The UK seems to stand alone against the whole Eurasian Continent, remember no one knew about Barbarossa. This will be a major hit for British morale. France falls in days and you're at war with yet another Great Power?