LINQPad version for quickly searching BDSP stationary and wild spreads. Slots are referenced from Poké Finder but aren't always correct, needs more research.

BDSP Stationary and Wild Calculator.cs

// Global settings to make things easier.

// Set these values for your gender ratio:
//    Fixed gender = -1
//     50 M : 50 F = 127
//     25 M : 75 F = 191
//     75 M : 25 F = 63
// 87.5 M : 12.5 F = 31
// Litleo line doesn't exist in BDSP so we can ignore the last case.
int GenderCompare = -1;

// Set this to true if you are searching wild slots.
// Slot calculation is not currently 100% accurate.
// This will also shift apparent delays by 84 since slot is generated 84 advances before EC.
bool SlotSearch = false;

// Set how many flawless 31 IVs your mon is expected to have.
int FlawlessIVs = 0;

void Main()
{
    // Initial state
    // s0 should be s1, s0 from CS
    // s1 should be s3, s2 from CS
    ulong s0 = 0x0123456789ABCDEF;
    ulong s1 = 0x0123456789ABCDEF;

    // Set this for how far you want to search.
    int maxAdvance = 500000;

    var rng = new XorShift128(s0, s1);
    for (int advances = 0; advances < maxAdvance; advances++)
    {
        GetStateWild(advances, rng);
        rng.Next();
    }
}

// You can define other methods, fields, classes and namespaces here
private readonly List<byte> slots = new List<byte> {20, 40, 50, 60, 70, 80, 85, 90, 94, 98, 99, 100};

private void GetStateWild(int advance, XorShift128 rng)
{
    var (s0, s1) = rng.GetState64();
    var output = $"{advance} | {s0:x16} | {s1:x16} |";

    if (SlotSearch)
    {
        var encslotroll = rng.NextInt(0, 100);
        var encslot = slots.FindIndex(z => encslotroll < z);
        output = output + $" {encslot:00} |";
    
        for (int advances = 0; advances < 84; advances++)
            rng.NextUInt();
    }

    var ec = rng.NextUInt();
    output = output + $" {ec:x8} |";
    var fakeTid = rng.NextUInt();
    var pid = rng.NextUInt();
    output = output + $" {pid:x8}";

    bool isShiny = false;
    ulong shinyXor = 0;
    int TSV = (int)((fakeTid >> 16 ^ (fakeTid & 0xFFFF)) >> 4);
    int PSV = (int)((pid >> 16 ^ (pid & 0xFFFF)) >> 4);
    if (PSV == TSV) // Shiny check vs a fake trainer.
    {
        var upper = (pid >> 16) ^ (fakeTid >> 16);
        shinyXor = (pid & 0xFFFF) ^ (fakeTid & 0xFFFF) ^ upper;
        isShiny = true;
        var shinySymbol = shinyXor == 0 ? "Q" : "T";
        output = output + $" {shinySymbol} |";
    }
    else
        output = output + "   |";

    int[] ivs = {-1, -1, -1, -1, -1, -1};
    var i = 0;
    while (i < FlawlessIVs)
    {
        var next = rng.NextUInt(6);
        if (ivs[next] == -1)
        {
            ivs[next] = 31;
            i++;
        }
    }

    var ivstring = "";
    for (i = 0; i < 6; i++)
    {
        if (ivs[i] == -1)
            ivs[i] = rng.NextInt(0, 32);
        ivstring += ivs[i];
        if (i < 5)
            ivstring += "/";
    }

    output = output + $" {ivstring} |";

    var ability = 1 + rng.NextUInt(2);
    output = output + $" {ability} |";

    var genderchar = "-";
    if (GenderCompare != -1)
    {
        var genderval = rng.NextUInt(253) + 1;
        if ((int)genderval < GenderCompare)
            genderchar = "F";
        else
            genderchar = "M";
    }
    output = output + $" {genderchar} |";

    var nature = (int)rng.NextUInt(25);
    output = output + $" {GameInfo.GetStrings(1).Natures[nature]}";

    // Add your filters here.
    if (nature != (int)Nature.Timid)
        return;
    if (!isShiny)
        return;
    //if (shinyXor != 0)
    //    return;
    //if (genderchar != "M")
    //    return;
    if (ivs[0] != 31 || ivs[1] != 0 || ivs[2] != 31 || ivs[3] != 31 || ivs[4] != 31 || ivs[5] != 31)
        return;

    Console.WriteLine(output);
}

Setup is:

1. Download/install LINQPad 7.
2. Download dev build PKHeX.
3. Press F4 in LINQPad and add PKHeX.Core.dll to first tab, and PKHeX.Core to second tab,
4. Change the language at the top to C# program, then copy/paste the script in verbatim.
5. Edit your param (seeds, advances, and filters).
6. Click run. Each line should be showing Advances, s0, s1, EC, PID, IVs, Nature.