Crypto assets consulting & development firm inacta (https://inacta.ch/) releases open source Dart and Python Bitcoin libraries for BIP32, BIP44, bech32, libbitcoin & BTCPay Server:

bitcoin_bip32: Dart library for Hierarchical Deterministic (HD) keys. https://github.com/Kolibri-POS/bitcoin_bip32
bitcoin_bip44: Dart library for HD wallets. https://github.com/Kolibri-POS/bitcoin_bip44
btcpay_client: Dart library for BTCPay. https://github.com/Kolibri-POS/btcpay_client
bech32: Dart library for BIP173 SegWit addresses. https://github.com/Kolibri-POS/bech32
pylibbitcoin: Python library for libbitcoin. https://github.com/libbitcoin/pylibbitcoin

Our developer Harm reflects on the lessons learned: https://medium.com/inacta-ag/it-starts-with-libraries-9f973f8e4601

Might be usefull for some of you, especially when you plan to develop a cross-platform mobile app (using Googles Flutter).