Dear Sister,
Salamun Alaikum.
Verse 4:103 makes use of the Arabic 'kitaban mawquta' which refers to times which are 'prescribed'. Although 'waqt' from which 'mawqutun' is formed means a state, fixed time or a period of time, the word 'kitaban' refers to a prescription or something that is enjoined / appointed / inscribed / what is written or ordained. Therefore, prayers are enjoined on believers at set periods of the day to be kept as diligently as possible.
However, it is also clear from the Quran that no expiation is prescribed for 'missing' prayers. Nor is there any prescription as to how missed prayer should be compensated for. However, the Quran does instruct believers not to abandon / neglect (ada'u) prayers (19:58-59). Furthermore, the Quran is replete with the instructions to 'establish prayer'; it makes it clear that even during war, prayer is to assembled albeit with precautions (4:102); that even during travel, prayers should be upheld as best one can (2:238-39) and even in fear, it can be shortened but not left (4:101). So establishing prayer is not a casual expectation by the Quran but one which is obligatory.
Keeping all this in view, one must make every effort to commit to prayers as best they can, God willing. There is indeed a relationship between one's commitment and the level of their 'imaan'. However, every effort needs to be made to engage. If a prayer is missed, there is no option but to move to the next prayer. However, one has the choice to include a missed prayer in the next one, ask for forgiveness, or to elongate the next prayer. There is no prescription here by the Quran.
Much of this is based on best endeavours and intentions. We must try to revolve our worldly affairs around our religious obligations as opposed to revolve our religious obligations around our worldly affairs. Of course, there will at times be a clash. However, we are only expected to do the best we can. Please see the following verse:
064:016
'So keep your duty to God as best you can / what you are able (Arabic: ma is'tata'tum), and listen, and obey, and spend; that is better for your souls. And whoso is saved from his own greed, such are the successful'
The Arabic word 'istata'a' means to be able, can or to be capable of. Therefore, given the above verse, one is only expected to the best they can whilst striving in the way of God.
I hope this helps , God willing.
Regards,
Joseph.