splitting notations

Well, there are different ways of doing things depending on what is going on with the music and how you think it would best be conveyed. From what you describe, I'd say the music is not meterless, but has a very clear meter of alternating 5 and 3, and so you should be aiming to actually convey that to the musicians reading reading, not obscure this fact. So the goal to me is not to create meterless music, but music that clearly denotes 5+3/4.

First, I assume you've ruled out 4/4 for some reason, but as a musician reading this, I'd question that too - unless there are tuplets that make this impossible, I'd likely rather just deal with rhythms that cross the barline than with changing meters. Or, since it only happens eight times and then the meter changes again, there's a good chance I'd want to see the alternating time signatures. But I'd have to see the music to really understand and weigh in on that. So for now, let's take it at face value you want to clearly convey 5+3/4.

A couple of different ways to do that, probably the most straightforward is to create the time signature as 8/4 but also set it to read 5+3/4 in the time signature properties, then click the note on beat 6 and do Edit / Measure / Split Measure Before Selected Note. Depending on how the music lays out and how you want the beaming to work, this might work right off the bat. Or you could not do the split but instead insert barlines "manually" in front of beat six. Another option is to notate it in 3/4 (again using Time Signature Properties to make it display as 5+3) and then use Measure Properties to add two beats to the actual duration of the appropriate measures. Probably other ways that could be used as well. It all depends on the specifics of the music and desired results. So attaching your score would definitely help us understand and assist better.