Not totally sure what your question is but my experiences are:

Relatively easy to gain points and move up the tiers. Once you are silver you get 25% extra miles for each flight so Gold can be achieved quite quickly. No distinction between tier miles and bonus miles

International Lounge access is fine, domestic access is limited (never totally sure what the rules are). Extra baggage allowance is very handy on domestic flights.

They don't really seem to upgrade but then a lot of their flights aren't completely full.

AI redemption was easy but taxes ex. UK are high. Haven't done a partner redemption.

If your going to fly AI at all it is worth being a member, for the 10kg extra baggage if nothing else.