I'll like to stay in the alstadt and that is closer to Koblenz
Statmitte. But does that mean if I had to get to the right side of the
river (say to Rudesheim), then would i have to take a train and change
at Koblenz HBF.
Yes, with the exception of trains 7:15 and earlier, or after 22:00. However, trains from Stadtmitte > Left-side-towns (like Bacharach, Oberwesel) trains are mostly direct.
All that said... you will likely encounter stairs at the smaller stations here and there too to reach your platforms. Use
https://www.bahnhof.de/en to research stations.
The Koblenz Hbf station area would be a more convenient place to stay than the old town, all things considered. Visiting the old town area from Koblenz Hbf is best done using Bus #1 from Hauptbahnhof E to the Deutsches Eck/Seilbahn (15 minutes, hourly service, see schedule page below.)
https://www.vrminfo.de/fileadmin/data/pdf/fahrplanbuecher/2025/1.pdf
All that said... If you do stay in Boppard, all is not necessarily lost...
You should note that from Boppard Hbf to points north (like Koblenz,) the trains use platform #1 - which should not be a problem for you because platform #1 is accessible without steps. Trips into Koblenz, or to Koblenz for the purpose of transferring to the Right-side railway for Braubach, Rüdesheim... or to the Mosel Valley railway... will be staircase-free at Boppard station. The same platform is used for trains arriving at Boppard Hbf from the south (from Frankfurt Airport, Bacharach, etc.)
Your dilemma from Boppard is traveling by train in the other direction - southbound. Koblenz > Boppard Hbf trains would mean using a staircase to exit the station in Boppard. However, you do not have to use trains for this route; Koblenz Hbf > Boppard is possible by direct bus, from either Koblenz Hbf or other Koblenz stops. Bus 670 makes this southbound run to Boppard every 30 minutes:
https://www.vrminfo.de/fileadmin/data/pdf/fahrplanbuecher/2025/670.pdf
Alternatively, you can take a southbound train from Koblenz Hbf to Filsen (25 minutes,) walk south to the ferry, then ferry across the Rhine and step off onto the Boppard waterfront.
For Boppard > St Goar, Oberwesel, Bacharach, the southbound journeys: Trains would mean staircases in Boppard at the beginning. Buses would be inconvenient/time consuming. So it might be wise to restructure your outings... do a reverse K-D cruise (10 am from Boppard - 11:20 St Goar, then 12:20 from there to one of the other towns) and return by train to Boppard (arriving at step-free platform #1.)
A second option to travel southbound from Boppard: First. cross the Rhine using the Boppard ferry, walk north to Filsen and use the train from there to Rüdesheim. Cruise, or cruise + train, back to Boppard platform #1 from there.