Added a BUY ALL button to the Research panel. It sits in the header next to the "COSTS PAID IN FLOPS" label. One click and it buys every research level you can currently afford across all tiers, starting from the cheapest. If you can afford multiple levels on the same tier, it keeps going until your FLOPS run out.
The button grays out automatically when there's nothing left to buy, so you can tell at a glance whether it's worth clicking.
You'll get a summary in the terminal showing how many levels were purchased and the total FLOPS spent.
Thanks to MAKAIROSI for the suggestion.