I started out with the roughneck maul like you, then got an x27 for Xmas and boy what a difference! I learnt lots of tricks, got quicker but just did more wood (12-15 cube a year) decided it was worth the investment in more axes....got an 8lb stihl/oschenkof maul for the 'unsplitables' and an x21, that increased doubled or tripled my speed. Then found the x21-27 gap too large, learnt to hang a head properly myself and got sent a true temper Jersey pattern head at just over 3lb.... that on 28" in easy stuff was sooooo fast. I only used a cheap haft though so eventually it broke. At this time I was busy so just went online and bought cheap (very cheap, spears and jackson) x25 and x23 ish copies. There is no 23, and the 2 axes aren't quite the right weight or length but you get the idea. Those axes are rubbish compared to the fiskars but I use those 2 most as they are the right size for most stuff. The x21 works BUT it's too short unless you're short. This makes it impossible to use without a block and dangerous if you don't bend your legs.. 2nd swing with it I hit my shin not the ground after a knot deflected the axe out sideways past my block... thankfully it was just stitches I needed.
Tbh, knowing what I know, if buying fresh now, I'd get the husky/gardena axe at x23 ish size, an x25 or s2800, an x27, and the stihl 8lb maul. Once you know how to split you'll use smaller axes most of the time.
Oh and if you're any where near se19 you're welcome to come and split some wood for me....I mean try the different axes. 😉