S&S ISA when in Japan

I am having a hard time getting my head around a lot of things, so apologies for stupid questions.

I am planning to apply for a spouse visa and move to Japan from the UK in October next year at the earliest, obviously visa approval-dependent.

I have a couple of cash ISAs, and stocks & shares ISAs (S&S around 17KGBP, cash about triple this). I am aware of the 5-year rule regarding remitting and foreign income, and intended to keep my cash ISA savings in the UK until the 5th year, and remit the following year. However I don't know what to do with the S&S ISA. Is the interest that accumulates but is kept in the ISA if I don't touch it the "dividends", and are they still taxed? As in, do they get taxed if I slip up and accidentally remit during those first 5 years? If I keep my S&S ISA open beyond those 5 years then when I eventually decide to cash in, is that when the dividends become tax-able?

For example, if I kept it open until the year 2050, and then cashed in would I pay tax on the interest earned in that final tax year only, or the total interest since originally paying in? How do you calculate the interest earned over the tax year if it is constantly changing?

I understand that these type of ISAs are designed to be long-term and so I don't know what to do with it now if I have to pay taxes on it in 5 years or possibly even sooner.

Also, I've only just become aware that if you are granted PR then the 5 year rule becomes irrelevent.