That is my understanding yes - only goods over 150 attract duty. You cannot use the IOSS system for goods over 150. You still have to use the ‘old’ methods with full customs declarations etc.
As for your VAT registrations in France and Italy - as long as you do not store goods in the EU and send just from the UK and the goods are always under 150 - then no I do not think you need those registrations (but please confirm with an accountant).
If you only sell via online marketplaces (such as Amazon) you dont need your own IOSS number as the marketplace will now collect the VAT and you send goods under their IOSS number.
You only need a personal IOSS number if you are selling from your own website AND want to send the goods pre-paid of VAT. There are alternatives to this too - such as using a company that will do the VAT part for you and you use their IOSS number.
You can also just send goods sold from your own website as DDU - and the buyer will be charged VAT before delivery.