I can't decide if i should do basic renovation to my bto (5rm) which est 40k or do a nicer design est 80k...My partner and i may only stay there max 8 years.

It depends on the condition of the renovation after 8 years before you may/may not factor in some plus points during the sales pitch.

8 years is still quite a long time imo, do something you'd like which will make you feel happy to be home! Comfort most important!!

Depends on what theme you would like for your house. For minimalist or industrial theme, maybe you do not need to spend so much. Personally I did a lot of DIY when I was renovating my BTO flat back in 2014-15. I only spent 13k.

Depends on how u would use the other 40k and if it will brings u more happiness than a house which is well designed. Some people find it important to come and feel refreshed / satisfied at the nice interior while some just treat it as a place to stay. to each their own.