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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.
2) Look into Lightning. Being able to receive Lightning is a bit of a headache, but look into Breez Wallet (https://breez.technology/) for that.
3) Like you said, custodial exchanges like Coinbase and Binance can do sub-dollar transactions internally, because those aren't really bitcoin transactions, but rather just internal database entries. Same as with Liquid only more so: as soon as you saved up a decent amount, withdraw to your own wallet. BlueWallet is another good wallet for Bitcoin (https://bluewallet.io)