You can try my EthernetWebServer library, which supports RP2040 + W5500 using either ArduinoCore-mbed or arduino-pico core.
Check this AdvancedWebServer example and you can either leave the html inside the .ino or split the html code to a separate .h file
LittleFS can also be used and you just load the html to a variable at the beginning, then use it as long as you'd like.
Check the similar examples written for ESP32 and ESP8266 and LittleFS
Here is the LittleFS_Mbed_RP2040 library for mbed core and LittleFS for arduino-pico core