That Alpkit bag does not look all that stable to me, get the feeling it'd do a fair amount of flopping around off-road.
Using two zip ties or thinner straps would be an option: one below and one above the stem. Should be as secure, if not more secure, than a single large strap.
I've not got any point of comparison, but the medium fuel pod doesn't seem particularly floppy to me. I use it on my 29er, as described above, and on my gravel bike with velcro below the stem. Normal cargo is tube, pump, wallet, phone, keys, gels.
The fore strap on the Restrap TT bag is double sided Velcro so you can move it around (the front of the pouch has a large patch of hook) you could also potentially add more double sided Velcro for additional security.
Failing that, a reusable zip tie would probably do the same job as the velcro.