Newbie here, also planning first grade. Just thinking out loud.
Isn't it possible, in any curriculum for kids to skirt by doing worksheets without full understanding? I mean, it's up to you as the teacher/parent to see your child's performance and manage that. We've used 1a & my child loves the Singapore books. I cannot imagine not having the HIG even though I can clearly add and subtract, I want to be sure I'm presenting it in such a way that it can be built upon for future lessons, the "singapore way" if you will. If you try it you've not lost anything being so young, and can just try something else later. I'm sticking with it until I'm 100% sure it's not working as it is an affordable, time tested program. We do "supplement" with xtramath.org, just because it's more fun than flashcards but it's certainly not an indication Singapore isn't enough. With any program there are days when it gets monotonous, so put the workbook away and do a game instead.
Since my daughter knows the phonograms, we're just doing AAS 1 (without tiles, she doesnt need them) as a combo phonics supplement/spelling program.