<!-- 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,
{
    Balance ret;
    isminefilter reuse_filter = avoid_reuse ? ISMINE_NO : ISMINE_USED;
    {
        LOCK(cs_wallet);
        std::set<uint256> trusted_parents;
        for (const auto& entry : mapWallet)
        {
            const CWalletTx& wtx = entry.second;
            const bool is_trusted{wtx.IsTrusted(trusted_parents)};
            const int tx_depth{wtx.GetDepthInMainChain()};
            const CAmount tx_credit_mine{wtx.GetAvailableCredit( true, ISMINE_SPENDABLE | reuse_filter)};
            const CAmount tx_credit_watchonly{wtx.GetAvailableCredit( true, ISMINE_WATCH_ONLY | reuse_filter)};
            if (is_trusted && tx_depth >= min_depth) {
                ret.m_mine_trusted += tx_credit_mine;
                ret.m_watchonly_trusted += tx_credit_watchonly;
            }
            if (!is_trusted && tx_depth == 0 && wtx.InMempool()) {
                ret.m_mine_untrusted_pending += tx_credit_mine;
                ret.m_watchonly_untrusted_pending += tx_credit_watchonly;
            }
            ret.m_mine_immature += wtx.GetImmatureCredit();
            ret.m_watchonly_immature += wtx.GetImmatureWatchOnlyCredit();
        }
    }
    return ret;
}
CAmount CWallet::GetAvailableBalance(const CCoinControl* coinControl) const