"Fortifying" House district races on such a scale would be next to impossible to pull off. You're giving them WAY to much credit, if all the theories surrounding vote fraud in the 2020 election are true, the way they pulled it off is that they basically added huge numbers of Biden ballots in safe Democrat areas where the entire process was under their friendly control and you'd only need a handful of people in each State to actually take fraudulent actions and cover them up.
To steal the US House would require basically a tenfold number of people in districts that are much less safely in their hands, greatly increasing the chance of discovery.
This is one of those advantages to the highly distributed nature of US elections, corruption can only really regularly impact State-wide races, but to organize a conspiracy wide enough to ensure they continue to control the House simply requires to large of a footprint, to many people, and to many moving parts to be reliable and for the risk to outweigh the reward. If they try and "fortify" any races in the 2022 midterms, don't look at the House, look to the Senate. They're much more concerned with maintaining and enlarging their control of the Senate rather than the House.
Why? From a strategic perspective for the last two years of the Biden presidency the Senate is going to be much more important than the House. The House as it stands now, is already a complete mess and dysfunctional, they can barely get the Dem coalition to pass anything. In fact, losing the House and burning the Progressive Caucus as much as possible in the loss would actually be in the longer term interests of the Dems, as it would allow them to rebuild with greater unity for the next time they control the House (seriously, this has been the most dysfunctional House majority in my lifetime, even Trump had managed to get some major legislation passed by this point). The Senate; however, is where they will need a stronger majority so they can try and push as many new judges into positions, ratify the host of internationalist treaties they are probably secretly negotiating right now, etc. Meanwhile, if the House goes to the Republicans it just means no legislation really gets passed... which is pretty much the same as now, but right now they can't get all the appointments they want or the treaties, especially the Globalist Watermelon ones, they want passed due to how narrowly they control the Senate and the power it gives the Blue Dog Dems.
Thus, if they try and cheat it's not going to be to keep the House, it's really not worth it and the present Dem coalition in the House is just a mess. No, if they cheat it's going to be to take real control of the Senate to enable as much Presidential rule by fiat as they can.