<!-- Describe the issue -->

Hi everyone,
We use bitcoin core with hundreds of wallets. The only efficient way we have found to do so is :
1. wait for any bitcoin-cli command to terminate before issuing another one
2. never issue any bitcoin-cli command unless tip update is 1
3. always unload a wallet after loading and using it

Now, we observ that bitcoin core systematically crashes every 20-30 minutes, then automatically restarts. And the problem is, it has to re-sync from about 0.997, which easily takes 15-20 minutes. During that time, the daemon cannot be used.

If we don't load/unload wallets, the daemon is stable and keeps to 1, no issue.

**System information**
Bitcoin Core RPC client version v0.20.0.0-ga62f0ed64f8bbbdfe6467ac5ce92ef5b5222d1bd
Compiled and run on Debian 9, on a VM with 16 mo memory. Launched that way : /bitcoind -daemon -walletrbf -conf=/root/.bitcoin/bitcoin.conf -fallbackfee=0.00001 -rpcworkqueue=128 -txindex=1

**bitcoin.conf :**
server=1
dbcache=16384
par=1
blocksonly=1
maxuploadtarget=137
maxconnections=16
rpcuser=bitcoinrpc
rpcpassword=xxxxxx
rpcallowip=127.0.0.1
testnet=0
txindex=1
walletbroadcast=1
txconfirmtarget=1

Please note that the same behaviour was happening with older releases (from 0.17.x).

Thanks for any help. Please ask for any needed information / testing.
Regards,
{
    AssertLockHeld(cs_wallet);
    
    std::map<COutPoint, Coin> coins;
    for (auto& input : tx.vin) {
        std::map<uint256, CWalletTx>::const_iterator mi = mapWallet.find(input.prevout.hash);
        if(mi == mapWallet.end() || input.prevout.n >= mi->second.tx->vout.size()) {
            return false;
        }
        const CWalletTx& wtx = mi->second;
        coins[input.prevout] = Coin(wtx.tx->vout[input.prevout.n], wtx.m_confirm.block_height, wtx.IsCoinBase());
    }
    std::map<int, std::string> input_errors;
    return SignTransaction(tx, coins, SIGHASH_ALL, input_errors);
}
bool CWallet::SignTransaction(CMutableTransaction& tx, const std::map<COutPoint, Coin>& coins, int sighash, std::map<int, std::string>& input_errors) const