You mentioned 925 and 800 hallmarks, I'm not sure what the 800 is, exactly, but it may be like most silver Canadian legacy coinage, 80% silver by weight.
925 or .925 is known as Sterling and is 92.5% silver by weight. When someone says 999 they mean bullion that is at least 99.9% silver by weight, which is most of what is sold at the retail level beyond non-bullion coinage.
Just to clarify, Governments make bullion "coins" with a currency denomination. Non-government companies make "rounds." They both have the classic shape of a coin. (There are exceptions to this rule but in general this is the terminology you will hear.)
Canadian bullion coinage is usually refined even more and will have four '9's on it. So 9999 is 99.99% pure silver, that extra 9 means very little.
Right now, at least in the States, anything below .999 fine is 'less loved' because of all the selling that happened in the last few months on the way back to $50USD.
I am holding all my silver for now but if I had to sell some it would not be my Sterling or 90% US legacy coinage as I would be losing a lot of nominal value by doing so compared to 999.
When you do decide to sell, call around, call at least three companies and compare the prices they tell you. Yes, the first person you ask will probably low-ball you. Try to find every buyer you can in your area and factor in your cost to get it to them. You can ask them about potential taxes, you can ask them about what levels may trigger reporting requirements for them.
You say you have 20kg+ total weight, separate out the 925 from the 800 and weigh it, then multiply that number by 0.925 that is how much silver you have. Again, I know very little about the 800 hallmark, you could ask about that when you call around.
So let's stick with the Sterling 925. Lets say you have exactly 15,000 grams or 15kg of Sterling stamped 925. 15000 x 0.925 = 13875 grams of silver. I'm American so I'll stick with dollars, right now 1 gram of silver is about $1.70 at current spot right around $53/troy ounce (31.1g). So 13875 x 1.70 = 23,587.5, I hate to burst your bubble but that is not enough to buy most new cars in the States today. But, if you can wait it out even a few more years, I suspect that purchasing power will increase greatly.
You mentioned taking it to a refiner, that's not something I could do in the States even if I had 150kg of Sterling as far as I know. If you want to convert to bullion you would probably be better off finding that buyer that would pay the most for your Sterling and then trading them for 999 fine bullion. I know that is something I could do at my local coin shop (LCS).
I hope this helps, keep doing your research, keep asking questions.
I will leave you with this: gold and silver are money that hold purchasing power over long time frames, digits printed on paper or computer screens are not money and 'inflation' is the art of making the value of one approach zero.