There are reviews on the tyres on the Decathlon site and some of them don't seem that positive. I personally wouldn't change them unless I had to though, its really down to whether they are an acceptable quality for you. Fitting a 7 speed cassette on a modern 8-10 speed freehub will require a 3-4mm spacer I think. Cheapest source is often aliexpress. You'll need to reset the derailleur high and low limiters etc. Your existing derailleur will be compatible unless you go for a cassette with a very wide granny gear. I suspect your existing granny gear is 32t on your freewheel and so ideal to match that for the new cassette. Amazon can be cheap for cassettes in my experience definitely worth considering them along other shops. Here in the UK it looks like you can get a altus derailleur, 7 speed cassette and spacer for about £25 delivered from amazon with the derailleur and cassette both being warehouse deals perhaps something similar is available at Amazon france. You may find cheaper especially with the January sales coming up.

Note if you end up with a spare freewheel hub 20" rear wheel, freewheel, derailleur and gear selector you basically have a kit you can sell on ebay etc as a single speed to 7 speed conversion perhaps aimed at those who bought a Hoptown 300/Tilt 100 and decide single speed isn't for them. Especially as they were parts originally fitted to a Decathlon bike and are actually superior to the derailleur and gear setup on the geared basic Hoptown which is a more basic 6 gear setup with non-Shimano derailleur. Just an idea.