This snippet can help you change your grub default kernel when booting up.

set_default_kernel.py

import subprocess
import os
import sys
import re

if os.geteuid() != 0:
    subprocess.call(['sudo', 'python3', *sys.argv])
    sys.exit()

output = subprocess.check_output(
    ("cat", "/boot/grub/grub.cfg")).decode("iso-8859-1")
output = output.splitlines()
display_options = []
options = []
digit1 = -1
digit2 = -1
displaynr = -1
for line in output:
    # +? denotes non-greedy search
    res = re.fullmatch(
        r"^(\t?)menuentry '(.+?)'.*$", line)
    if res:
        displaynr += 1
        if res.group(1):
            display_options.append(("\t" + res.group(2), displaynr))
            digit2 += 1
            options.append((res.group(2), f"{str(digit1)}>{str(digit2)}"))
        else:
            digit2 = -1
            display_options.append((res.group(2), displaynr))
            digit1 += 1
            options.append((res.group(2), str(digit1)))
    else:
        res = re.fullmatch(
            r"^submenu '(.+?)'.*$", line)
        if res:
            digit2 = -1
            display_options.append((res.group(1), -1))
            digit1 += 1


ljust = 3 if displaynr > 9 else 2
for i, option in enumerate(display_options):
    if option[1] != -1:
        print(f"[{option[1]}".ljust(ljust) + f"] {option[0]}")
    else:
        print(" " * ljust + f"  {option[0]}")


option = int(input("\nPick one: "))
selection = options[option]
print(f"Your choice: {selection[0]}")

grub = open("/etc/default/grub", 'r')
lines = grub.readlines()
for i, line in enumerate(lines):
    if re.match(r"^GRUB_DEFAULT=.*$", line):
        lines[i] = "GRUB_DEFAULT=\"" + selection[1] + "\"\n"

grub = open("/etc/default/grub", 'w')
grub.writelines(lines)
grub.close()

subprocess.run(("update-grub"))