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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Dunno. Who would have though "increment by one" could be so complicated? https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#child-key-derivation-ckd-functions If I was doing this, I would read the publicly available source code at https://github.com/iancoleman/bip39 (javascript), or one of the Python wallets, and correlate the code with that section of the BIP32 document
Don't use intermediaries that rip you off, or any intermediaries in the first place, they are unnecessary in bitcoin. Use open source, decentralized payment processors, like BTCPayserver: https://btcpayserver.org/