My theory is Joe is a laid back guy, and pretty passive in the relationship (clear from how he acts in vlogs). They were perfect for eachother because they both ticked each others boxes - good looking, well off, in the right social circles etc. But ultimately they lacked any kind of real connection and chemistry.
Joe is clearly serious about his job and when the India opportunity came up they probably had a conversation about what it meant for their relationship, I wouldn’t be surprised if Joe went into that convo thinking they may even break up at that point. Niomi surprised him when she said she wanted to join him, especially as what she actually wants is to settle down and have a family in the UK. They probably discussed delaying their settling down plans for a couple of years so that they could live in India and therefore Niomi was making a big compromise for him. He probably hadn’t felt ready to propose at that point but because of family pressures, thinking it was the right thing to he went ahead and did it. The fact the whole process of choosing and making the ring was in India, it kind of indicates his thinking - that India would be a big part of their lives for the next few years so seemed appropriate.
Joe was clearly devastated that India got cut short, and even though Niomi said she was too I think she was secretly happy about it. They were now able to bring forward the wedding planning (all driven by Niomi) and part of Niomi may have thought that actually they could settle down sooner than she had expected. They were probably on completely separate pages and not communicating properly at this point in early lockdown. Moving in with Freddie should have been a red flag for Niomi because Joe knew wherever they lived in the UK would be temporary before they were off again. Niomi was naive, she had thrown herself back into work in the UK, started full on wedding planning and probably thought they were here to stay for the long haul. When discussing careers and Joe wanting to go abroad again, the cracks started showing and they realised they wanted different things and had different expectations of their lives together.
With Niomi saying it’s now time to ‘put herself first’ etc I think she’s referring to having to make compromises for Joe like moving to India and feeling like he didn’t make any big compromises for her like considering not going abroad. Joe probably then felt annoyed because he was making compromises everyday, not eating what he wanted to because Niomi was controlling, being filmed etc. It all made for an unhealthy relationship that ultimately wasn’t going to work. Joe was probably keeping a lot of these feelings to himself and then it all came out one day which made Niomi describe it as ‘traumatic’, she probably had no idea - living a little happy bubble because on paper she had everything she wanted again. No cheating involved in my opinion.