There are two problems with playing classic Risk with 7 players: map size and game length.
A lot of Risk strategy revolves around players having continents and/or territory. On the classic map, there are 5 viable continents (Asia is not a position that can be held for any duration as a player either gets invaded or wins soon after taking it; the other 5 are). On top of this, the standard risk map has only 42 territories, and you need players to be able to hit 12 territories for territories to start mattering for reinforcements (0 - 11 territories are counted as equivalent to 9 territories for starting reinforcements).
Classic Risk is best with 4 to 5 players, because there is at least one viable continent per player and 8 - 10 territories per player. Even with 6 players, at least one player will be left homeless and/or will get eliminated in the early game, so unless that's the vibe you're going for, you need a bigger map.
This is easy to fix. There are plenty of maps you can find online, or even some variants of risk that have been made to support more players. The Lord of the Rings Trilogy Edition came with a 9 continent map with 63 territories (it was a bad map for other reasons; in particular the lack of bottlenecks), you could find some on sites like this: https://www.warzone.com/Maps, or build your own. You could even stick two classic Risk boards next to each other (although this might need a few more connections between boards than just Kamchatka). As long as it has continents, asymmetry, and bottlenecks, it will probably be fun.
This one is much harder to fix. Risk is a game with completely serial turns, so each player you add makes the game longer. 6 players is already pushing it. If you have a group of people willing to pay all day, that's fine, except that players who get eliminated early will have cleared their schedule for nothing, which feels bad. There are basically 3 options to fix this:
Play asynchronously. Find an online or mobile platform that will let you play with your friends at a pace of a turn a day or so and stretch the game out over weeks. This will likely include the ability to play on a custom map as well.
Find a variant that is more parallel. You could take classic risk but have everyone reinforce at once. Then, during the attack phase, have everyone declare one attack at once, and then roll for them simultaneously. Finally, everyone does troop movement at once. It would be a little chaotic, but you'd at least keep things moving. Here's another suggestion: http://drmuey.com/?do=page&id=18.
Play a different game. Diplomacy is in the same genre but designed for 7 people at baseline and is built with simultaneous turns. It has a number of different maps that have been made for it for a number of player counts (I've seen maps go up to 50 players, enabled by the parallelism). And, it was designed to be played by mail, so there are plenty of online platforms for it to play asynchronously (https://vdiplomacy.com/, https://webdiplomacy.net/).