Which lighter seats are going to allow me to use a normal seatbelt? You can't really use one with any bucket set.
Still, if you're going to start doing it, you may as well do it properly.
Rear cage is around £400, harnesses maybe £100 and I already have some seats that are in date. The front portion of the cage can be added later for £350, but at least the rear would put me in good standing for buckets and harnesses.
90KG is a lot to lose when you need to keep all interior trim in place (although carpets and sound deadening can be lost). Best chance is swapping front seats for some really light items, ditching aircon, polycarb the rear window and dumping the carpets/sound deadening.