The problem with trying to make 'new' size jump cores- 7 or 8 collar JS for example- is they do not quite know the how. Outside of ComStar shipyards, no one has the modeling programs, before the 50s no one had built a core that did not go in a current model JS.
When I was in the Army I had TMs for my vehicles, and with that guide book I could repair most problems. I could even tell you how a lot of it worked. But I could not take that book and go design a whole new armored vehicle.
But those two things are immensely different in the structural side of things. No offense meant, but when / if you were repairing tech in the army, you probably weren't half as qualified for building those things you repaired or inventing improved or variant versions of those than a specialist engineer working for a specialist company trying to wrap his head around how to go from an Invader or a Star Lord to a 8-collar JumpShip.
Being a repair tech doesn't mean you can build anything, not even things you would be repairing every day. If I would hand my car to
any repair technician and tell him "make me a copy", he would laugh at me. It's not a matter of handing him a second spare car either, he simply has neither the tools nor the knowledge.
But if I hand a 1990s Chinese car corporation a 1998 Chevrolet Spark or Daewoo Matiz and tell them "copy it", they will take less than 5 years to do so. It actually happened.
And it's not unreasonable to assume something similar for JumpShip technology. All JS technology must exist archived digitally. So you can send all the blueprints, simulations, data and figures to your research and development institutions (universities, corporate R&D) and tell them "wrap you heads around it" and then let them come up with something. In BattleTech, JumpShip technology is perfectly scalable. So going from an Invader or Star Lord to a Monolith is a matter of scaling up the energy output, the size of the core etc. Of course the devil will be in the details and you need the industry to actually deliver the parts, but that is something very different as not being able to come up with plans.
No, for me the solution to "LosTech" always was "there was no option to do produce this" and not "it could not be re-thought again". Why is that the case? Well, I can think of two answers.
First, you can give all data to your research and development institutions, but at some point you need actual parts to start building new JumpShip classes. And since it costs huge amounts of money and you're bleeding high tech gear left and right (constant warfare will do that to you), you need to prioritize. In 1945 the German armament industry had made significant advancements for 20 years, but they couldn't just flip a switch and build all the shiny stuff, because they were loosing production facilities and raw resources left and right. Building jet engines is a problem, when you're short on rare materialy, it's utterly pointless, if you lack jet fuel and run your trucks on wood gas generators.
Second, when your military capabilities are shrinking and you loose units monthly, raw economical power daily and bright minds every year, it might sound feasible to invest in optimized military logistics, including DropShips and JumpShips, but how could you accomplish that? We just have to take a look at the former USSR and its successor countries. Even Russia is still decades behind Western countries in most industrial and social sectors, including medical capabilites and quality of life. In some areas of this huge country, people are even worse off than they were during the 1980s and we're almost 30 years after the end of the Cold War and there were numerous initiatives to get all sorts of aid into the country. Except for a few paragon projects, the Russian military is only slowls climbing back to capabilities on par with its most likely opponents. And we're talking here of the biggest and most powerful Successor State, looking at Ukraine, Belarus, Kasachstan or the numerous smaller countries and we see no such abilities to offensively prevail in a war with their neighbours (including Russia).
Third, Russia suffered a devastating brain drain during the 1990s and 2000s and still is. If you're top educated and from Russia or Ukraine, you will take that offer from Harvard or Oxford or Sorbonne or Munich if and once it comes. Because it will only come once.
And this is ComStar at its finest. Operation HOLY SHROUD might have been about the destruction of Successor State knowledge and might have involved a lot of killings. But Scientists are not necessarily hardcore attached to their Successor State, not in a generation that still had known an interstellar community of research and scientific advancement. I would bet, ComStar made several advancements on top scientists telling them "do what you want, but do it on Terra, we need to rebuild mankinds ancestral home and save humanity from the bloodshed and destruction that is already here".
And then of course, ComStar has another ace up its sleeves. The moment Successor Lords start sending their precious data on soon-to-be LostTech around the ether to build data caches and restart their research programs, they will probably send this data via HPG. After all, HPGs are fast, highly capable and extremely reliable, since they're run by the one and only neutral actor: ComStar. And since ComStar dictates HPG usage rules and can basically decipher every message, it would have been easy for ROM to alter LosTech data. It doesn't take much to alter data on KF-technology to make it completely useless and probably hazardous. Just alter the last digit of some coefficients or constants and no jump core will work in this universe. Or add some picometers to core processor nodes and watch them burn...
tl;dr
It's the complexity of the task and the multitude of actors involved combined with the scarcity of ressources, not the impossibility to come up with plans and prototypes, which made technological progress or even recovery impossible during the darkest days of the Succession Wars era. Once your precious tech is gone, you need peace or at least outside help to recover. Cringy analogy: If all of your world is the USSR and its successors, there are no helping hands and Harvard keeps stealing your brightest minds, good luck recovering in time, before the Clans come.