This assumes that the 1929 recession was special and could only be weathered by Roosevelt, but the economy was recovering in 1934 and the May 1937 recession is both after Garner's reelection would have been decided and possibly caused by the fiscal stresses of the Second New Deal that Garner opposed. In 1936 Garner gets the same (probably unearned) credit for the (probably natural) economic recovery that FDR did and coasts to reelection unless Long is alive to act as a spoiler. A stock market crash under Hoover and a recovery under Garner is not a recipe for a Republican being elected in 1936. If the 1937 recession does still happen it's a recipe for a Republican in 1940, but Republican convention delegates in 1940 were not willing to nominate an isolationist to the point that they nominated someone who hadn't actually run in any primary.
It also assumes that the interwar Republican Party is the caricature Democrat propaganda paints them as, but when you look at the actual candidates they're not. In 1936 Garner is running against Alf Landon unless something upsets the Republican convention. Alf Landon might have been an isolationist in 1936, but he was a proponent of aid to the UK and France in 1939. The other serious contender was William Borah, but the party establishment undertook great effort to ensure a brokered convention in which he would lose and I don't think that changes. Other potential nominees were Frank Knox (definitely interventionist), Frederick Steiwer (died before the war but was interventionist in WWI to the point of resigning a state senate seat to join the army), and former president Hoover (isolationist, but I doubt that given his baggage he would have been the party establishment's second pick).
Congress passed lend-lease in March 1941 when it had banned loans to belligerents in 1936 because isolationists, both in congress and among the voters in intervening elections, changed their minds. Roosevelt took advantage of this, but he did not cause it. Adolf Hitler did.