My advice? Fire your commander for embezzlement. She's the one who blew a huge wad of your unit's resources on her own vanity. Arrest her if you can find a legal jurisdiction that covers your people. Once she's gone, either sell off the Achilles and shuttle, or sell off all your mechs. At your unit's size, you're nowhere near big enough to tackle both ground and space contracts, you need to pick one venue and focus on it. If your ground force REALLY wants an assault ship, you can sell the Achilles for an Avenger or two or an Intruder, and have enough cash left over to upgrade all your mechs and maybe even hire an additional lance or two.
For shuttles:
If you want to throw a boarding party, get a Battle Taxi.
If you want to move cargo or carry out infantry drops, get a Mk. VII Landing Craft.
If you want to land cargo on any terrain, get a K-1 DropShuttle. Ignore what anything/anyone says about docking collars. That is nothing but fluff and is completely unsupported in the actual game.
If you want to impress employers and other VIPs, get a K-1, some plywood boards, a few rolls of nice carpet, and a couple rooms' worth of furnishings from SpaceKEA. Use the plywood and carpet to create modular floor panels, and now you can fancy-up a few compartments in your DropShuttle to make a bigwig more comfortable while still conveying the message that their money is going to be spent on practical things like defending their holdings, or blowing up their enemies.
If you want to move mechs or combat vehicles, get a DropShip.