Though on the topic of older designs, I've been considering some of the Lyran Jihad era mechs, like the Mjolnir and the Eisenfaust, which seem like machines that frontline units wouldn't really be too interested in.
It’s been three-quarters of a century, so I think that’s fair. I think MarauderD’s comments on Commandos, bugs, and the holy trinity are also good, as are Colt’s on older designs getting updates in the Recognition Guides.
Field Manual 3145 and its RATs may be your best guide. In fact, there’s only one mech battalion of Timbuktu Theater Militia left (the Florida TTM), stationed on Halifax. It’s veteran, at 80% strength, and has a C-grade equipment rating. That gives it an “11” on average on the associated RAT, which yields the Razorback-10S, Nightsky-6T, Bhargest-4T, and Zeus-X4. Stuff like the Mjolnir and Eisenfaust show up lower on the table, so they are certainly viable. So do the Uziel-8S, P-Hawk-7S, Starslayer-3Dr, Griffin-6S, Hatchetman-7S, Clint-6S, Hunchback-5S, and VT-5Sr Vulcan, Stilletto-6S, Wolfhound-5, Scarabus-9T, Commando-7S, Stinger-7S, Storm Raider-R3, and Dart-4S. More recent/advanced stuff like the Firestarter-M2, Arctic Fox, Thunder Fox, and Gauntlet are within range higher on the table. Heavier stuff on the table is generally too slow for pirate hunting duty, but the Thanatos, Usa, and Scourge (with a perfect roll) are possibilities.
Of course, RATs are not definitive. The Bushwhacker doesn’t appear at all on this Lyran RAT, but obviously exists on their rolls, for example.