How can I make this 2006 Acer TravelMate more usable?

Realistically, there may not be a worthwhile path beyond a RAM upgrade and the SSD you already installed. Even the lightest Linux setup will struggle if the machine has only 2GB of RAM and a 32-bit-era CPU. A used laptop from the last decade with 8GB of RAM would provide a much larger performance improvement for relatively little money.

The SSD was the right upgrade and should make startup and application loading noticeably faster, but it cannot overcome the limits of a very old single-core or low-end processor. Keep the desktop environment light, disable unnecessary startup services, and avoid running several applications at once.

The biggest limitation is probably the age of the processor and the small amount of DDR2 memory. If it only has 2GB, upgrading to the maximum supported RAM—likely 4GB—could help a little. Check the exact motherboard and CPU specifications first, since the chipset may limit the upgrade.

For basic offline tasks, simple text editing, and older lightweight software, it may still be usable. Modern web browsing is the difficult part because current sites are heavy and browsers use a lot of memory. A lighter browser and carefully chosen extensions might help, but streaming video and complex websites will probably remain frustrating.

Some TravelMate models from that era used socketed processors, so a compatible Core 2 Duo, such as a T9xxx-series chip, might be an inexpensive upgrade. However, check the BIOS, chipset, cooling system, and supported CPU list before buying anything. The improvement may be modest compared with the cost and effort.