you're probably right. it's feasible with a few cuts. It would be bigger than 3050U at well over 250 pages as-is
TRO: Succession Wars - 92 Mechs
TRO: 3050U - difficult to count because I'm not sure how many upgrades they'd do from the previous volume. Definitely 16 Clan omnis
Figure the Unseen that didn't make the cut into TRO: SW, plus any new machines. Call it...20 IS?
TRO: 3055U - 38 IS, 4 Clan Omnis. Questionable whether the 3 IS (Nexus, Raijin, Grand Crusader), 10 Clan phoenix, or 25 Solaris VII mechs would be included
I'd say hard "no" on the Solaris machines, since they require Tactical Operations. Yes to the nuSeen ComStar and Clan Second-Line
TRO: 3058U - 19 IS, 18 Clan (not counting the SL mechs. Most are in TRO: SW save the Cestus)
Include the Cestus. 20 IS, 18 Clan total.
TRO: 3060 - 30 IS, 26 Clan
I'd say save this for TRO: Civil War. Yes, many of these designs debuted around the time of the Great Refusal, but you're seriously pushing page count and risk making new players' eyes gloss over.
So, my total is 129 machines. That's a LOT. If we cut out the 3058U 'Mechs, we're at 91, which is right about TRO: SW. Perhaps don't sweat the exact delineations between eras and put all the 'Mechs from 3058U, 3060, and 3067 in TRO: Civil War. This would make a further theoretical TRO: Jihad* be 3075, 3085, and some of Prototypes, and TRO: Republic be the rest of Prototypes, 3145, and 3150.
Move the vehicles, fighters, and space craft to online supplementary volumes. This keeps the in-store focus on the giant stompy robots, but for those who want more, they can still get everything.
* And maaaaaaybe consider another name for this volume.