Is this the easiest way from Cambridge to Bath?

If you're worried about catching a train connection, then buy a flexible ticket (anything except an "advance single"), which you can do on gwr.com or any other train line's website. This will allow you to miss a train and have your tickets still be valid on a later one.

As for alternative routes, you could catch the bus from Cambridge to Oxford and then head directly to Bath. However I have never made this journey so others might be better suited to tell you whether this is a good route.

Unfortunately the Varsity Bus from Cambridge to Oxford no longer exists in a recognisable form. The route has been split into two at Bedford, and somewhat slowed down to gain passengers.

A through ticket always routes you via Kings Cross and gives you more than adequate time for the transfer on the Circle/Hammersmith and City Lines.
There is very adequate signage at both KC and Paddington to direct you. If the train was late into Kings Cross (on a through ticket) such that you couldn't make the connection then your ticket would be automatically valid on the next Paddington to Bath train.

Although you can't do it as a through ticket (would need separate tickets) I think the easiest way is a Thameslink train to Farringdon then the Elizabeth Line (the Thameslink train will be headed for Brighton).

That is a bit slower than the train to KC.

Or the Greater Anglia train to London Liverpool Street then the Elizabeth Line- the slowest of the three options. That is almost always the cheapest option if on separate tickets.

I don't usually give much credit to Rome2Rio because it screws up so magnificently and so frequently, but in this case they are fairly close.

Great Northern train from Cambridge to London Kings Cross is fastest, and faster than the Thameslink into St Pancras International.

Kings Cross and St Pancras share a tube station (Kings Cross St Pancras) where you can get a Circle Line tube to Paddington, or a Hammersmith & City tube to the same place. Or if you have a little time there os a bus to Paddington, the number 205 from stop "A" across the Euston Road in front of St Pancras which is also the easiest bus 205 stop for Kings Cross.

Or taxi.

Or, if you had taken the slower Thameslink to St Pancras and stayed on one extra stop to Farringdon you could make a simple change to the Elizabeth Line to Paddington.

As is often the case in London there are many ways to achieve a transport goal and no clear winner.

Once at Paddington, yes it is the GWR train to Bath although others on the forums advocate for sitting on a bus.

Thank you so very much for all your advise and also about Rome2Rio!

the bus from Cambridge to Oxford (formerly known as the X5) is no more. It is not a through service any more, the component parts cobbled together (I believe that it is not only change in Bedford but also change in St Neots as well) are simple town buses instead of the lovely leather seated coaches with free wifi on the former X5. And it takes forever if you do pull it off.

Even the bit between Oxford, Kidlington, Buckingham, Milton Keynes and Bedford (unless they force a change at Milton Keynes) calling itself the X5 is now usually a simple town double decker with no facilities when I pass it on the road.

Take the train.

As someone who has used the X5 pretty recently I can confirm that it does still have free wifi, even if it is "just" a town double decker. The X5 is actually also a Megabus route.
Not that anyone was seriously suggesting it as a route to Bath, as it is about 4 hrs 30 minutes from Cambridge to Oxford. And the 905 is through Cambridge to Bedford, both routes being worked exclusively from Bedford depot.
Then a further 90 minutes to Swindon on the now hourly or better S6 bus, then 90 minutes to Chippenham on the 55 and a further hour to Bath.
Not a route a tourist is at all likely to do, but one that gives a very different route to the train. Clearly train wins on speed.
An all day oddysey.
A chief glory of the £2 bus fares has been the chance to do bus routes you wouldn't do normally on cost grounds, just to explore.
Next week I break the train journey from Cumbria to London up with the bus from Wigan to Crewe via Warrington and Northwich, just for something different, new scenery. Then train Crewe to London, then the through ex Green line bus London to Reading.
Two weeks ago I did a similar trip with bus Crewe to Stafford, in between trains.