rigora

No suffix in sight. Rigor- is the whole root. It is easy to read an -or- in there, partly because the ending looks like the agent nouns some national languages build that way, and partly because Esperanto learners get trained to hunt for affixes. Esperanto has no official -or- suffix at all, and rig- is not a root, so there is nothing to find. It came in from Latin as one piece.

Three near neighbours, and they are not interchangeable.Severa is severe or stern and describes a person's manner: a severa patro is hard on you. Strikta is tight, first physically and then by extension: a strikta regulo leaves no slack. Rigora is the one about standards not being relaxed, and it is the word scholarship and method use: a rigora pruvo is a proof with no gaps in it, a rigora ekzameno is one that tests properly. Where English would say rigorous research, Esperanto says rigora esploro and means it approvingly.

The family. Rigoro is rigour as a quality, rigore is the adverb, and rigorismo is rigour taken to the point of being a doctrine, which is usually a criticism. Notice that none of these is a moral judgement by itself: preciza is its friendly relative and rigora is what you get when precision is enforced rather than merely admired.