Stuart,
I'm glad you're interested in this trip... I haven't yet been to Salaspils, but Rumbala is a very moving and haunting place. You could do the trip on public transportation -- you can pick up a number of buses to get to Rumbala forest from very close to where the cruise ship shuttles drop off. Google will give you transit directions in English or the Riga Public Transit website has an English option in the upper right hand side of the page.
https://www.rigassatiksme.lv/en/for-riga-guests/public-transport-in-riga/
The trouble is that I'm not seeing a clear route between Salaspils and Rumbala on public transport. It looks like it's about 2km and theoretically walkable, but I don't have any experience with that. I'd recommend using the Bolt taxi app, and hiring someone to take you to both places. Bolt works like Uber, so it's tough to estimate the fare but I can't imagine the total would be more than 20-30 Euros.
I don't know how long you intend to spend at the monuments, but the Riga Ghetto museum, which is very near the Central Market might be a good way to get some additional context. It's open every day except Shabbat and they have English speaking guides available.