Using TFT and Neopixel at the same time

I don´t have idea, but you can explore it with the ILI9341_t3 library, by the way, the load of bmp is very slow.

Teensy 3.6 and FT813

I have a teensy 3.6, with a FT813 TFT 5"@GD3_SDIO, with a 8x5 WS2812 matrix and your idea is posible

#include <SPI.h>
#include <GD3_SDIO.h>  //THX RndMnkIII

#include <Adafruit_GFX.h>
#include <Adafruit_NeoMatrix.h>
#include <Adafruit_NeoPixel.h>

File archivo;
SdFatSdio SD;

unsigned long start;

//Neopixel
#define PIN 21
#define NUMPIXELS      40

Adafruit_NeoMatrix matrix = Adafruit_NeoMatrix(5, 8, 1, 1, PIN,
  NEO_MATRIX_COLUMNS + NEO_MATRIX_BOTTOM + NEO_MATRIX_LEFT,
  NEO_GRB + NEO_KHZ800);

Adafruit_NeoPixel pixels = Adafruit_NeoPixel(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);
int BrilloMX = 150;

void setup()
{
  pixels.begin(); // This initializes the NeoPixel library
  matrix.begin();    matrix.setBrightness(BrilloMX); 

  GD.begin();
  GD.cmd_setrotate(0);
  
  SD.begin();

  JPGSlider();
}

void loop(){}

char nombre[32];
int ID = 1;
long previousMillisBMP = 0, previousMillisNEO = 0;
long intervalBMP = 100,  intervalNEO = 30;

void JPGSlider()
{
  GD.wr(REG_PWM_DUTY, 0);
  while(1){
  GD.wr(REG_PWM_DUTY, 30);
  unsigned long currentMillisNEO = millis();        
   if(currentMillisNEO - previousMillisNEO > intervalNEO)
    {previousMillisNEO = currentMillisNEO;
     Cylon1(100,0,0); pixels.show();}
    
  unsigned long currentMillisBMP = millis();        
   if(currentMillisBMP - previousMillisBMP > intervalBMP)
    {previousMillisBMP = currentMillisBMP;   
     start = millis();
     snprintf(nombre, 30, "V%03d.jpg", ID);
     String str(nombre);
     archivo = SD.open(str);      
     GD.cmd_loadimage(0, 0);
     GD.loadSDIO(archivo);
     CargaJPG();  ID++;   if (ID>=161){ID=1;}
    }
  }
}

void CargaJPG()
{
   GD.Clear();
   GD.Begin(BITMAPS);  
   GD.Vertex2ii(0, 0); 
   
   GD.ColorRGB(0,255,0);  GD.cmd_number(730,465,21,OPT_CENTER|OPT_SIGNED,millis() - start);              
   GD.swap();
}

Video: BMP slider MEGA vs Teensy 3.6