There are easy ways to counter the ships you mentioned, without disabling blueprint uploads. The server and the blockbehavior config can help a lot.
- To disable megaminers, you can limit the max number of outputs linked to a single computer, as many servers already do (2K outputs on Brierie, 250 outputs on GenX - just examples). This will keep salvage arrays at a reasonable size. The array simply won't work and pop up an error message if there are more groups than allowed.
- To get rid of OP megaships, just implement a lower mass limit. You can't forever protect the new/returning players from large, well-armed ships, eventually there will be people who no-life the game to get the goods and slap together a million mass doombrick. There are players like that, for example Elwyn Eternity had Joelbrisco, Brierie has SnakeRandal who still uses unimaginative doombricks despite the server allowing blueprint uploads. Hell, if someone really wanted their ships and has a strong enough computer, they can just save them piece by piece as templates and put them together that way. I say that as a victim of said unimaginative oversized doombricks. You know as well, since you're veterans yourselves that everything past 200K mass is just a sluggish turretfest with boring-ass gameplay as it's impossible to maintain a good thrust/mass ratio or turn rate at that point, making fights practically turret matches where the first to miraculously turn their bow towards the enemy and bring their massive spinal mounted weapons to bear, wins.
Disabling blueprint uploads, which I believe is a core feature of the game, only delays the appearance of your definition of OP ships, does not stop it. It does however, make inexperienced or returning players vulnerable for a longer period against griefers already present, when they arrive to the server, forcing them to spend days or weeks trying to design a competent vessel from the ground up, all the meanwhile being easy targets for the early birds on the server who already took their time to do so, massively shifting the power balance towards the already established factions - and as time passes, this will get only more prevalent.