<!-- 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,
{
    
    for (ScriptPubKeyMan* spk_man : GetAllScriptPubKeyMans()) {
        
        
        if (spk_man->SignTransaction(tx, coins, sighash, input_errors)) {
            return true;
        }
    }
    
    
    for (unsigned int i = 0; i < tx.vin.size(); i++) {
        
        CTxIn& txin = tx.vin[i];
        auto coin = coins.find(txin.prevout);
        if (coin == coins.end() || coin->second.IsSpent()) {
            input_errors[i] = "Input not found or already spent";
            continue;
        }
        
        SignatureData sigdata = DataFromTransaction(tx, i, coin->second.out);
        if (!sigdata.complete) {
            input_errors[i] = "Unable to sign input, missing keys";
            continue;
        }
    }
    return false;
}
TransactionError CWallet::FillPSBT(PartiallySignedTransaction& psbtx, bool& complete, int sighash_type, bool sign, bool bip32derivs, size_t * n_signed) const