I’ve got two phase 2 cars and have changed both thermostats and changed the coolant on both. Neither needed any bleeding, just fill up to the max level, run until hot, allow to cool and top up if necessary.
If you suspect a blown head gasket look for mayo like gunge inside the oil filler cap and/or in the coolant reservoir. Also might be worth checking that your gauge isn’t U/S. You can pick up spares for a tenner on eBay. I’ve had my car stop getting up to temp twice. Once was the thermostat failed open so it never warmed up unless stationary and the second time was the gauge itself. 10 min job to swap gauges.
2001 GTV Twin Spark - Aerokit, teledials, Ragazzon quads, MTEC discs, DS Performance pads
2000 Spider Twin Spark - 17” multispokes, colour matched mirrors, OEM luggage rack, Ultra Racing strut brace, Fox quads, Zender fuel flap cover