Since you're talking hypotheticals: no matter what is banned, whether something limited like bumpstocks or something big-time like all semi-auto firearms or accessories those bans will unquestionably be ineffective at first. There are just too many of whatever out there right now for anything to make an immediate difference. I've always thought that whatever is done would be done for two generations down the line - for our grandchildren. As I think 757 posted a week or so ago, if we take positive, meaningful action, it'll work 'eventually.' Nobody wants to hear 'eventually,' everybody wants 'right now,' but given the sheer volume of what's on the streets and in the still legal retail pipeline, we have to live with 'eventually' or accept the apologist argument that it's already too late; it won't work, and criminals and crazy shooters will always have access to them. But if we take action, eventually they won't.
For the practical matters, we could give dollar for dollar tax credits for the buybacks and a long grace period before criminal penalties for possession kicked in. As an extra incentive to turn stuff in, items seized or found one way or another during the grace period wouldn't get the $ tax credits. I think the whole thing could be structured so that otherwise law-abiding citizens who happened to own things that were going to be illegal would have plenty of time and notice to be able to avoid any negative interactions with the criminal justice system entirely.
What would I do in your hypothetical? I realize that assault rifles are just semi-auto rifles in macho costume. A relatively benign-looking Ruger Mini 14 with a big clip is functionally the same as a decked-out AR-15. Ban easy stuff like bumpstocks and silencers. Make concealed carry permits difficult to get, not only with regs like drivers licenses but with some sort of proven need like was the case in VA when I still lived there (remember the Daily Progress printed a list of everybody in the city who had a permit and the gun attached to it back in the early 80s - that raised a big stink - but the list wasn't real long, either). Serious capacity limits for semi-auto long guns - say 8 rounds- assuming, of course, that the court in your hypo limits its ruling to the constitutionality of 'assault guns' and declines to address the capacity issue. Require some sort of attaching mechanism that slows down clip changing. Penalties for altering guns or clips or adding accessory devices to increase capacity, rate of fire, or speed of changing clips. Ban of new manufacture hi-cap semi-auto handguns. For the millions and millions of existing pistols with higher capacity, sheesh, that's a can of worms, huh? Maybe have a buyback on the clips themselves and allow the existing handguns with replacement lower capacity clips - big market opportunity for somebody to make reliable 8 round clips that fit an 18 round handgun.