Is the LANKAQR payment thing usable for tourists or do I need cash everywhere

LANKAQR is growing fast but it is tied to local Sri Lankan bank and wallet apps, so as a short-stay tourist you generally cannot use it unless you open a local wallet. In practice: cards (Visa/Mastercard) work at hotels, bigger restaurants and supermarkets in cities and the south, while cash is essential for tuk-tuks, buses, small eateries, markets and village guesthouses. So carry rupees. To avoid your card being blocked, tell your bank your travel dates before you fly (set a travel notice), and always choose to be charged in rupees, never your home currency, at the terminal.