https://iancoleman.io/bip39/ has been around for several years, popular, and trusted. Just check the "Show Entropy Details" box at the top and you can input your own entropy including dice rolls and/or playing cards.
This tool, like most other tools, is taking the entropy from user and simply converts that to words. What OP is looking for is a way to combine that input with another source of entropy (generated by computer) and then convert the result to words.
I can have this site generate a random one of 15 words. (Obviously after cloning it from GitHub and moving it into offline computer.) Then presumably I could simply go into "Show entropy details" and add my dice rolls to the entropy until I get up to e.g. 24 words. Anyone here welcome to critique this idea.