Solve problems using bitcoin smart contracts

Been trying to ask this to few people on different platforms but nothing was helpful so trying here and will keep trying because I don’t even know what else can I do to learn more and contribute.

What will be the best way or resources to refer if someone has an idea and want to implement it using bitcoin smart contracts?

For example: If I want to create a decentralized insurance product which might even need external data so won’t be completely decentralized but trust minimized, non custodial, non kyc and better than present solutions. Users will have to regularly pay some amount which should be locked in some multisig setup and used in a way that it gets interest so maybe provide liquidity in joinmarket, create a lending/borrowing thing using discreet log contracts or something else, these profits can be used to cover life and release amounts after verification on claims

How would I approach solving this problem using bitcoin?

What are best resources to learn more about discreet log contracts, it’s examples?

Could CHECKSIGADD be used on new ECDSA multisignatures instead of CHECKMULTISIG?

The proposed Taproot soft fork (BIP 342) defines the new opcode OP_CHECKSIGADD to verify Schnorr multisignatures whilst OP_CHECKMULTISIG (and OP_CHECKMULTISIGVERIFY) are disabled for Taproot spends (SegWit version 1). Could we use the OP_CHECKSIGADD opcode for verifying new SegWit version 0 ECDSA multisignatures? Obviously you don’t get the same batch verification benefits if the individual signatures are ECDSA but it would still be more efficient to use the counter system of CHECKSIGADD?

For an explanation of how the opcode CHECKSIGADD works see this presentation from James Chiang. Or see Tapscript BIP 342.

"The opcode 186 (0xba) is named as OP_CHECKSIGADD"

"OP_CHECKSIGADD is functionally equivalent to OP_ROT OP_SWAP OP_CHECKSIG OP_ADD"

Bip39 tool – BIP32 Extended Private Key vs BIP32 Root Key

In the bip39 mnemonic tool there is three sections :

  1. Mnemonic : we input our words or a BIP39 seed and the tool compute for us the BIP32 Root Key
  2. Derivation Path: there is a BIP32 Extended Private/Public Key
  3. Derived Addresses

Questions :

1- If i want to use Electrum wallet and I have already a wallet. Which key should I have to import ? What is the difference if I import the BIP32 Root Key or if I import the BIP32 Extended Private Key ?

2- Should I use BIP44 instead of BIP32 to import my wallet in an Electrum one ?

When using bitcoin-cli I get an error regarding fallback fees when trying to send regtest Bitcoin. How do I resolve this?

I have been doing an assignment on Bitcoin Core 0.20. Now every time I try to do a transaction:

Apples-iMac:bitcoin apple$ bitcoin-cli -regtest sendtoaddress bcrt1qd3jck0lv87yuk25tk3xy5rqq67apwasee874xk 25 
error code: -4
error message:
Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable -fallbackfee.

Can some one help me – what should I change? Please give a step by step guide as I am a complete newcomer.