<!-- 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);
    mapKeyBirth.clear();
    LegacyScriptPubKeyMan* spk_man = GetLegacyScriptPubKeyMan();
    assert(spk_man != nullptr);
    LOCK(spk_man->cs_KeyStore);
    
    for (const auto& entry : spk_man->mapKeyMetadata) {
        if (entry.second.nCreateTime) {
            mapKeyBirth[entry.first] = entry.second.nCreateTime;
        }
    }
    
    std::map<CKeyID, const CWalletTx::Confirmation*> mapKeyFirstBlock;
    CWalletTx::Confirmation max_confirm;
    max_confirm.block_height = GetLastBlockHeight() > 144 ? GetLastBlockHeight() - 144 : 0; 
    CHECK_NONFATAL(chain().findAncestorByHeight(GetLastBlockHash(), max_confirm.block_height, FoundBlock().hash(max_confirm.hashBlock)));
    for (const CKeyID &keyid : spk_man->GetKeys()) {
        if (mapKeyBirth.count(keyid) == 0)
            mapKeyFirstBlock[keyid] = &max_confirm;
    }
    
    if (mapKeyFirstBlock.empty())
        return;