<!-- 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);
    
    CKey seed_key;
    seed_key.MakeNewKey(true);
    CPubKey seed = seed_key.GetPubKey();
    assert(seed_key.VerifyPubKey(seed));
    
    CExtKey master_key;
    master_key.SetSeed(seed_key.begin(), seed_key.size());
    for (bool internal : {false, true}) {
        for (OutputType t : OUTPUT_TYPES) {
            auto spk_manager = std::unique_ptr<DescriptorScriptPubKeyMan>(new DescriptorScriptPubKeyMan(*this, internal));
            if (IsCrypted()) {
                if (IsLocked()) {
                    throw std::runtime_error(std::string(__func__) + ": Wallet is locked, cannot setup new descriptors");
                }
                if (!spk_manager->CheckDecryptionKey(vMasterKey) && !spk_manager->Encrypt(vMasterKey, nullptr)) {
                    throw std::runtime_error(std::string(__func__) + ": Could not encrypt new descriptors");
                }
            }
            spk_manager->SetupDescriptorGeneration(master_key, t);
            uint256 id = spk_manager->GetID();
            m_spk_managers[id] = std::move(spk_manager);
            SetActiveScriptPubKeyMan(id, t, internal);
        }
    }
}
void CWallet::SetActiveScriptPubKeyMan(uint256 id, OutputType type, bool internal, bool memonly)