For a mountain bike, I'd use Slime. One department-store bike I had came with some non-brand Chinese Slime-like flatstop material in the tube. I rode it for 3 weeks before removing the tire. I found 3-4 places where goatshead thorns punctured the tire and the flatstop stopped the flat.

That was informative. I don't think I would have tried Slime. But, seeing it work so well, now I would.

I think the reason Slime gets bad reports is that it's only good for lower-pressure tires like mountain bikes. At higher pressure it sprays more than clogs. In your case, it should work well.

For my pavement bikes, I replaced the tires with Bontrager hardcase. $40 each. (I've heard good things about Vittoria Randonneurs for $30. I might try them next time.).

The next time I have some cheap department-store tires, I'd like to try Mr. Tuffy tire liners. They create a hardcase barrier. For about $7. I'd like to see if they work.

There are also thorn resistant tubes with thicker walls. I tried some on my pavement bikes but being narrower tires, the thicker rubber reduced the available air cavity, giving it a harsh ride (especially in combination with stiffer hardcase tires). That might not be as much of a problem for a mountain bike with wider tires at lower pressures. TR tubes might be a good combination with Slime (and/or stiffer tires).