Basics, the frame trick is what actually makes the far jab into ultra link possible. So unfortunately it can’t get any better than that.
Varleran, generally speaking, combos that mix ex uppers and ultras are a waste of meter. If your goal is damage and you score a jump in roundhouse then you can go right into low strong, fierce headbutt, ultra, for over 500 damage. That’s half the inputs of the combo you mentioned and costs nothing. And even if you’re not at the point where you can hit confirm off of the jump in itself you can do low jab, low strong, fierce headbutt, ultra, for right around 470 without spending any meter either.
The primary draw of long combos with Rog is not the extra damage (as you know, scaling kills the damage). Rather, it’s the extra stun. And that makes finishing a long combo with his ultra (a move that does no stun) practically pointless. Just having Rogs ultra puts you in a favorable position during a match. It lets you control a lot of space. When you tack on an ultra at the end of a big combo you’re basically exchanging all that control for a very small one time boost to your damage output, which is very unwise.
DISCLAIMER: If doing an ultra after a big combo wins you the round then none of the above points matter and it was a great idea.
Anyway, back on topic. A far more practical setup for linking into Rogs ultra, yet very underused, is off of a DSB. All versions of the DSB are +10 on hit, making it a 3 frame link with the frame trick. You need to do the DSB from around midscreen with charge buffering to have charge for the ultra though. It’s not something that I would pull out terribly often, but once in a while it works great as long as you don’t telegraph that you’re trying to set it up or get predictable with it. It’s the same as throwing out a seemingly random ex dsb into low short into fierce headbutt every now and then. DSBs are always a gamble though, regardless of the setup. Still, when it pays off, it pays off big.