From Fulfillment Company to Self-Managed

Hi! I actually have a question for fellow e-commerce store owners here.  I've been running an online t-shirt company for almost 3 years now. Basically, its just a graphic design tee dropshipping store. In those 3 years, I was using a 3rd party fulfillment company called Red Stag here in the US for bulk orders - and for the most part that has been working well. For smaller orders, I do the usual direct to customer dropshipping set-up, and if its bigger orders from retailers, I work with Red Stag.

However, fairly recently (give or take the last 3 months) I was keen on seeing if I could actually start doing customized shirts, which would require me to drop my supplier (as they don't offer than option) and take everything in-house. I was wondering what software (for tracking, inventory, etc) are y'all using for your stores that are self-managed? 

I could still stick with Red Stag but from a financial pov, it only makes sense with bulk orders. Would love some input and feedback regarding this - any comments at all is highly appreciated. Tysm!