Hi Tanya,
I would stick with just using the Monte Carlo. If you want to make it more stringent you can try increasing the number of tests and passing percentage or make the validation acceptance criteria better. I’ve found 20 and 80 works well though.
The walk forward is a more advanced tool(but that doesn’t mean better!). I have used it in the past but I no longer do.
For robust strategies I would recommend the combination of a good amount of data with plenty trades, OOS and Monte Carlo.
I only trade M15, H1 and H4 and I typically like the use 5 years data for M15 and H1 and about 10 years for H4. Can use more but I wouldn’t use less.