<!-- 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,
else {
            error = strprintf(_("Error loading %s"), walletFile);
            return nullptr;
        }
    }
    if (fFirstRun)
    {
        
        walletInstance->SetMinVersion(FEATURE_LATEST);
        walletInstance->SetWalletFlags(wallet_creation_flags, false);
        
        if (!walletInstance->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
            walletInstance->SetupLegacyScriptPubKeyMan();
        }
        if (!(wallet_creation_flags & (WALLET_FLAG_DISABLE_PRIVATE_KEYS | WALLET_FLAG_BLANK_WALLET))) {
            LOCK(walletInstance->cs_wallet);
            if (walletInstance->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
                walletInstance->SetupDescriptorScriptPubKeyMans();
                
            } else {
                
                for (auto spk_man : walletInstance->GetActiveScriptPubKeyMans()) {
                    if (!spk_man->SetupGeneration()) {
                        error = _("Unable to generate initial keys");
                        return nullptr;
                    }
                }
            }
        }
        walletInstance->chainStateFlushed(chain.getTipLocator());