Congrats on your baby, mama! Honestly, day 17 is still super early and common na mababa pa ang supply because your body is still regulating, try power pumping or feeding more frequently muna bago mag-supplement, kasi the more you express, the more your body produces, pero if baby is losing weight or you're really stressed, there's nothing wrong with mix feeding either while you're building supply up. For me, what helped talaga was drinking tons of water, eating well, and making sure baby was latching properly (sometimes the issue is latch hindi talaga low supply), so maybe check with a lactation consultant or your OB muna para makita kung talagang low na or if there's a latch issue you can fix, kasi once na-establish mo ang supply now, mas mahirap na i-increase later if you go full formula agad.