I am curious about if there are multiple ways to say jes, since jes seems a bit too hard for circumstances where I wish to say "sure" or "whatever", where I agree, but don't care too much about what I am agreeing with.
I am also curious if words like the french word "Ouais" or "yeah" exist, to avoid saying Ja during these casual conversations.
Aside from jes, in spoken language you will often hear something similar to yeah (jee?), probably due to English influence or similar, although if one were to simplify the Esperanto word it would probably become something like that also.
Another possibility is to say simply bone or o kej, which fits in certain contexts, such as to express contentment with or acceptance of a stated idea. Other contexts might allow for mi konsentas or konsentite (to express explicit agreement), and in yet other contexts, kompreneble or klare (to express understanding), although these of course are now more complicated expressions rather than ones even more informal than jes.
You can indeed say ja often times as you suggest (works especially when contrasting the agreement with a perceived possibility for disagreement), and you can combine the two for enthusiasm or emphasis by saying jes ja!.
Oh, and since you were asking specifically about a softer way to say yes where jes is too strong, you can always add modifiers like nu and similar (nu, jes, mi supozas, eble...).
Krom jes, en elparolata lingvo oni ofte aŭdas ion similan al yeah (jee?), verŝajne pro angla influo aŭ io tia, kvankam se oni simpligus la esperantan vorton, ankaŭ tiu verŝajne iĝus io tia.
Alia eblo estas diri simple bone aŭ o kej, kiu taŭgas en kelkaj kuntekstoj, kiel por esprimi kontentecon pri aŭ konsenton kun dirita ideo. Aliaj kuntekstoj povas permesi mi konsentas aŭ konsentite (por esprimi specifan konsenton), kaj en eĉ aliaj kuntekstoj, kompreneble aŭ klare (por esprimi komprenon), kvankam tiuj kompreneble fakte estas pli komplikaj esprimoj anstataŭ esprimoj eĉ pli familiaraj ol jes.
Oni ja povas diri ja laŭ via sugesto (funkcias speciale por kontrastigi la konsenton kun kredita eblo de malkonsento), kaj oni povas kunigi la du pro entuziasmo aŭ emfazo per diri jes ja!
Ho, kaj ĉar vi demandis specife pri pli mola maniero diri yes kiam jes estas tro forta, oni ĉiam povas aldoni vortojn kiel nu kaj similajn (nu, jes, mi supozas, eble...)
I wouldn't say they are multiple ways to say “yes”, but there are several ways to agree, express affirmation. As a general answer, though, nodding and repeating jes isn't avoided in Esperanto, as it may be in some dialects of English (then probably “yeah” is a go-to nodding word). But, to make you have some choice next time:
This question (and the answers so far) seems to cover a few different topics.
First - the basic way to say "yes" in Esperanto is indeed jes, as you know, and I would encourage people to use that word for agreement for a few reasons - one of which is that the sound of the word cuts through differently than "yeah" or "uh huh" or "ja" - and is distinct from "ne". Also, as an international language, you want to make sure that what you're saying is understood, so having one simple and clear word is a good thing.
Second - don't confuse people carrying their baggage into Esperanto with what Esperanto is. Intermediate French speakers from English speaking countries often answer "yeah" or "uh huh" when speaking French, but that doesn't mean that these are French words.
Third - saying yes and no are often culturally dependent, so a good Esperantist needs to become aware of this. In the part of the US where I live, "thank you" often means "yes", but in other parts of the world it means "no." For that reason, always answer "jes dankon" or "ne dankon" - never just "dankon" to a yes/no question. Nodding your head yes is pretty widespread but is also not universal. Another example of this is that in Japan "jes" often means "I am listening" and not "I agree."
But practically speaking, as has been mentioned, there are many ways to express agreement without saying the word jes - bone, klare, mi konsentas, kompreneble, volonte mi faros, mi akceptas vian proponon and the list could go on and on.
I'm surprised that nobody has mentioned the somewhat sarcastic malne.
