Is the lack of phonetic pronunciation in the various forms of roihm just something that I have to deal with?

romham \r-oh-m\
but this looks like it should be pronounced \ro-vam\, since the mh here makes a \v\ sound.

romhat \r-oh-at\
but looks like \ro-vat\

roimhe \riv-eh\
I've seen it said that oi can make \oh\ or \ih\, and then I often seen it pronounced somewhere in the middle so this would be like \rov-eh\

roimpi \rim-pee\
same issue as above... It'd be great if I could just pronounce "oi" as something between \oh\ and \ih\.

romhainn \roy-n\
looks to me like \ro-vahn\ since mh makes \v\ sound and ai makes \ah\ sound.

romhaibh \r-oh-v\
looks like we could just drop the aibh at the end and get the same pronunciation...

rompu \rompuh\
I'm fine with this one

I realize that the Irish language isn't perfectly phonetic, but still I just wanted to post this here and see if anyone can share some enlightening words with me. Thanks for your patience with me!!

YOu are going to learn nothing at all if you don't learn the International Phonetic Alphabet first, I'm afraid.

Just looked into what "International Phonetic Alphabet" is and I'm not gonna lie, looks like a major headache / side quest...>.<

There's no need to learn the phonetic alphabet first. You'd never tell a student of Spanish or French or German to to learn it first, so Irish need not be any different.

As for "Roimh", it tends to be close to: roe-um, roe-ut, ri-veh, rim-pee, roe-ing, rove or roe-iv, rome-puh.

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I think it is helpful to have some familiarity with phonetics and the IPA when learning a new language if you want to properly master the sounds of the language. Otherwise you are likely to end up like >95% of people who learn Irish - having terrible diction.

When you see two short vowels together in the spelling, typically only one of them is pronounced. The purpose of the other is to simply indicate that the adjoining consonant is broad or slender. In the case of 'roimhe', the i is pronounced and the o is not pronounced, it simply being used to indicate that the r is broad. (Although in contemporary Irish, all word-initial r's have become broad, so from a phonetic viewpoint, you could say that the o now serves no real purpose.)