Are the "umbles" etymologically related?

The -umble words generally share a common ending in the frequentive verb suffix -le. Aside from that, I haven't found any direct etymological connections. (Note that it is regular for -b- to be inserted between m and l in this context.)

However, the similar sounds may have influenced the development of similar meanings, and for some words there were other phonetic variants that did not survive, possibly because these variants did not have the same sound-symbolism. Many of these words seem to have unclear etymologies (often supposed to be "imitative") that can't be definitively traced back all the way to Proto-Indo-European.

I'll try to go in order from the earliest attested words to the most recent.

tumble:

c. 1300, "to perform as an acrobat," also "to fall down," perhaps from a frequentative form of Old English tumbian "dance about, tumble, leap." This is of unknown origin but apparently related to Middle Low German tummelen "to turn, dance," Dutch tuimelen "to tumble," Old High German tumon, German taumeln "to turn, reel." (Online Etymology Dictionary)

Wiktionary is less cautious and gives a reconstruction for a common Germanic ancestor *tūmōną; if this is a valid reconstruction, it means the evidence points towards the vowel being originally long ū which was later subject to shortening in some words. Germanic roots with long /uː/ before a labial consonant like /m/ could develop in various ways in English; this handout on the Great Vowel Shift from Anthony Kroch's website summarizes them.

The modern form shows a short vowel /ʌ/ which generally corresponds to Middle English short /u/. However, the Oxford English Dictionary says that a variant with a long vowel /uː/, spelled "toumble," also existed in Middle English (there is also an attested post—Great Vowel Shift spelling "toomble"). In German, the two vowel length variants tummeln and taumeln became differentiated by meaning, but in English the variant with a long vowel simply became extinct. It's possible this was influenced by the existence of other words with similar meaning that ended with the rhyme -⁠umble (I can't find any words in standard Modern English with the rhyme -⁠oomble).

stumble:

c. 1300, "to trip or miss one's footing" (physically or morally), probably from a Scandinavian source (compare dialectal Norwegian stumla, Swedish stambla "to stumble"), probably from a variant of the Proto-Germanic base *stam-, source of Old English stamerian "to stammer," German stumm, Dutch stom "dumb, silent." Possibly influenced in form by stumpen "to stumble," but the -b- may be purely euphonious. (Online Etymology Dictionary)

The Oxford English Dictionary specifies that the vowels in stammer and stumble are believed to reflect different ablaut variants of the ancestral Proto-Indo-European root. I don't see any way this vowel could be connected to the Proto-Germanic long ū in the root of tumble.

mumble:

early 14c., momelen, "to eat in a slow, ineffective manner" (perhaps "to talk with one's mouth full"), probably frequentative of interjection mum. The -b- is excrescent. Meaning "to speak indistinctly" is from mid-14c. (Online Etymology Dictionary)

The supposed root, the interjection mum, seems to have no established etymology and is thought to be imitative.

fumble:

mid-15c., "handle clumsily," possibly from Old Norse falma "to fumble, grope." Similar words in Scandinavian and North Sea Germanic (Swedish fumla; Dutch fommelen) suggest onomatopoeia from a sound felt to indicate clumsiness (compare bumble, stumble, and obsolete English famble, fimble of roughly the same meaning). (Online Etymology Dictionary)

Again, the etymology is obscure and sound symbolism is invoked to explain the origin.

grumble:

1580s, "complain in a low voice;" 1590s, "make a low, rumbling sound," from Middle French grommeler "mutter between the teeth" or directly from Middle Dutch grommelen "murmur, mutter, grunt," from grommen "to rumble, growl." Imitative, or perhaps akin to grim (adj.). With excrescent -b- as in mumble. (Online Etymology Dictionary)

"-umble" appears to have a phonetic symbolic function in a number of terms as explained by Hans Marchand in the following extract:

From Phonetic symbolism in English word-formation.

Final symbols: rime derivation.

• 77. Rime plays a great part in language, .... But its part is obviously more important than has hitherto been recognized. In this chapter I shall deal with rime as having played a part in the derivation of symbolic words. Words may be derived from others as their riming counterparts, distinguished from them by the initial only.

-umble:

• is symbolic of indistinct humming or rumbling noises, as in mumble 1362, rumble 1384, obs. humble 'hum' 1384, jumble 1529, orig. 'grumble', obs. drumble 'mumble' (1579 to 1596), grumble 1586 (see initial /gr/). Tumble 1300 is the frequ. of OED tumbian 'tumble, make somersaults'. With the basis of 'tumbling movement' it at- tracted stumble 1325 (see initial /st/; the OED supposes Scand. origin), fumble 1508 (see initial /f/).