<!-- 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,
{
    if (!IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
        WalletLogPrintf("Cannot add WalletDescriptor to a non-descriptor wallet\n");
        return nullptr;
    }
    LOCK(cs_wallet);
    auto new_spk_man = std::unique_ptr<DescriptorScriptPubKeyMan>(new DescriptorScriptPubKeyMan(*this, desc));
    
    auto old_spk_man = GetDescriptorScriptPubKeyMan(desc);
    if (old_spk_man) {
        WalletLogPrintf("Update existing descriptor: %s\n", desc.descriptor->ToString());
        {
            LOCK(old_spk_man->cs_desc_man);
            new_spk_man->SetCache(old_spk_man->GetWalletDescriptor().cache);
        }
        
        auto old_spk_man_id = old_spk_man->GetID();
        for (bool internal : {false, true}) {
            for (OutputType t : OUTPUT_TYPES) {
                auto active_spk_man = GetScriptPubKeyMan(t, internal);
                if (active_spk_man && active_spk_man->GetID() == old_spk_man_id) {
                    if (internal) {
                        m_internal_spk_managers.erase(t);
                    } else {
                        m_external_spk_managers.erase(t);
                    }
                    break;
                }
            }
        }