Correct. There's plenty that can be done to prevent people voting, and the Republicans have got it down to a tee in places like Georgia, and other places that matter. Things like voter cross-check (purging the voter rolls), and then eliminating provisional ballots*. They've been doing it since Bush "won" in 2000. The Democrats were, as always, a lot slower on the uptake and - like the Labour party in the UK - expend most of their energy and tactics on stopping the left of their own party getting votes (see Sanders v Clinton for example). Things like mail in ballots are a red herring, there is very little fraud in those areas as anyone in the UK would know. However, an accusation of fraud is often as good as actual fraud when you want a vote challenged thrown out, which is why Trump mentions it continuously. The reason for targetting mail in ballots is because they generally come from three cohorts. Those professional middle class types that are well organised and work in cities, plus lower class ethnic minorities that are very time poor and whose employers simply wouldn't allow them time to vote in person. The older generation still prefer the tradition of in-person voting unless they are disabled, and of course those with a disability also utilise them. Traditionally, all of the above would be a Democrat majority. That is not to say that everyone who is a Democrat uses mail in, and vice versa, just that there's a very obvious majority, so targetting the whole is fruitful.
Trump's biggest issue with a mid-term will be voter turnout. More people will turn out with a grievance than with lukewarm support. That's the same for any president, but will probably be a large factor this time, as he's lost a large part of his popularity (again, not unusual). There is nothing he can do to prevent states from holding their vote, but there is plenty he can do afterwards to challenge and indeed fix the result. It will start with full scale accusations of fraud when he loses, and then Republican states not declaring and so on. Votes get thrown out until the correct result is returned. However, I think it will beyond his control, as the turnover required will be too great.
*the volume of votes not counted in certain Republican states is staggering.