Mathematically, lump sum is better.
Psychologically, spreading it out may be better. For example if the market is trending down, you will have bought more shares, cheaper (win). If it trends upwards, you will still make money - just a bit less of it (which may still feel like a win).
Personally, I would stick to the math and do lump sum in your situation. It's statistically better. If you make many such small-advantage financial decisions many times, these small advantages snowball into a big advantage.