asses vs assess: 'asses' is the standard plural of 'ass', which has been around since Old English, and pluralization doesn't change word stress. As the plural marker -es follows the stress, the vowel is reduced to schwa. 'assess' comes from Middle French 'assesser', which had stress on the end syllable. That got adopted into Middle English as 'assessen', with the -en being an infinitive ending (as it still is in German and Dutch). Removing that ending, as it would be when conjugated, you get the stem 'assess-', with stress on the end. Because that vowel is stressed, it isn't reduced to schwa.
these vs theses: 'these' goes back to Old English, and while the details aren't hugely important, suffice it to say that there were various processes that caused the /θ/ (<th>) and /s/ to become voiced as /δ/ and /z/. 'Theses' is from Ancient Greek (filtered through Latin), and is probably a relatively modern loan, vaguely from the Enlightenment if I had to guess. The Greek spelling is θέσεις (theseis). The Greek letter <θ> is strictly the voiceless 'th' sound (the sound in 'thing', NOT in 'this'). The first vowel is the 'bee' vowel /i/ because it's stressed, while the second one is also /i/ because that's how <ει> has been pronounced in Greek since vaguely the Roman era. English, like all European languages, has its own tradition of how to pronounce words from Greek and Latin that have diverged a fair bit from how they were originally pronounced, giving weirdness like this.
trough vs through: -ough is notoriously terrible, but it wasn't always this way. Back in Old English, these words ended in either -g or -h. -h was the ending sound of Scottish 'Loch', while -g was basically the same sound, but voiced. As you know, these sounds do not exist in English today, and so they generally either became silent or shifted to the next closest thing, often /f/. This depended on the exact phonetic context and was generally a mess, though I'll do my best to untangle things. 'trough' was 'trog' or 'troh' in Old English, while 'through' was 'thurh'. If I had to guess, I'd say that it went silent in 'thurh' because it was preceded by an /r/, and so it could be dropped while still being recognizable as the same word (note how you can easily still recognize the word didn't even if you don't pronounce the /t/). This wasn't the case in 'trog', and so it became an /f/ as the next closest sounding consonant. The 'loch' sound /x/ and /f/ both produce some raspy rush of air, so it's not completely weird.
though vs thought: this one is a bit messy. 'though' strictly speaking comes from Old English 'þēah', which we might expect to get an -f ending. However, it was conflated with Old Norse 'þó', which dropped the ending consonant and changed the vowel. A huge amount of Old Norse vocabulary entered English during the late Old English period and displaced quite a lot of native English vocabulary, including pronouns. 'them', for instance, isn't actually a native English word, but rather is from Old Norse. 'thought' comes from Old English 'þōht', where as before with 'thurh', the sound could be dropped without impeding word recognition. The evolution of the vowels is a whole hot mess due to English having one of the most complex vowel systems in the world, so I'm gonna just leave that as 'people talked and fucked the vowels up'.
though vs thorough: I don't think there's that much weird about this one? 'thorough' is from a corruption of Old English 'thurh' (through) into 'thuruh', which came to be used as an adjective and gained initial word stress that caused the vowels to evolve differently. It's not that goofy, all things considered.
thank god, we're done with the <gh> disaster. The Scots really had it right when they decided to just keep it.
stranger vs strangler: this is predictable. The <g> in 'stranger' is reduced to 'dʒ' (the consonant in "Joe") because it's followed by <e>, reflecting a stage of palatalization in Middle French where the word originates (originally Latin 'extraneus'). This isn't the case in 'strangler', so it behaves as normal. Oh, I guess the vowels are different; like I said, English vowels are a disaster. So, 'stranger' was borrowed from Anglo-Norman, a weird dialect of Old French originating from the Normans that conquered Britain. It was divergent from more standard Old French in a few ways, and in this case, 'stranger' comes from Anglo-Norman 'straungier'. This turned into a long /a/ in Middle English (the vowel of 'father'), which the Great Vowel Shift turned into vowel we have today. 'Strangle', on the other hand, comes from Old French 'estrangler' and entered English with a short vowel. So, 'strange' originally had a long /a/ while 'strangle' had a short /a/, and those both evolved into different sounds that are spelled with the same letter because English is insane.