The crutch you need depends which problem you want to solve. If you select your own deep rooting lettuces and aim for good structure and nutrition, you will get to head maturity under most conditions (plus if you care for predatory beetles/fireflies, so that there is a low initial density).

If you want to stick to the system you have, we need to get creative to find a solution. I found that good varieties plus foliar feeding works in most circumstances here – but this is Arion slugs. Deroceras can be a pain if they sneak inside the heads and just nibble where the midrib curves upward. I found them a lot harder to control with nutrition, but a lot easier to control by predators. Limax specializes here to eliminate heat stressed plants the day after it occurred. It often beheads plants just above the point where lignification sets in.