Mart, thanks for this, to think we worry about rain…
Lettuce is the main difficulty with germination in hot weather as it can lie dormant. But NOW is season for sowing endive and chicory (with you at least, almost too late here) and also brassica salads which germinate happily in warm soil or compost. So it really should not be a problem! Any of rocket, mizuna, mustards, pak choi, tatsoi, leaf radish, kale and so on, also other salads such as spinach, chard and coriander.
You are right that there is no time to lose and I am sowing my tunnel brassica salads this week, or next at the absolute latest.