Hello from Floreat side of Perth (few more days).
Be mindful of the 100hrs EASA compliant (ie not training w FI) PIC for licence issue (could be bit less for the conversion flying or whole 30hrs module, which won't be needed if you have Aus CPL). You may or may not have that doing CASA commercial.
'frozen atpl' is CPL with ATPL theory credit. Unlike CASA or FAA etc systems, most people in Europe, going for airlines etc, avoid repeating hasle/cost of courses. Which have to be through formal ground school provider even distance study with brush up classes/revision etc.
With Aus CPL, you can do 'training as required' for commercial. Due to the cost of MECIR in Australia, unless you plan to return here, not worth doing it. In Europe they normally do twin rating together with IR before or after CPL module, depending on one's plans/proficiency/experience. Importance of NDB/ADF stuff compared to FAA system for practical flying, but frequently used in Australia, so would still help. Some do their twin IR in Spain, some Brits look down on Spanish IR standards.
You can sign up even NOW for ATPL course for EASA. All you need is showing them ICAO PPL or higher, some IDs, etc. Nothing onerous.
CATS (Cranfield Aviation, now based in LUton and Gatwick) is probably cheapest in UK for DS ATPL course. Other 'major' is Bristol GS, CAPT (mainly for heli guys, but do A as well). Varies with revision delivery, though, as well as locations.
Anything else, you should do plenty research yourself from 'sticky' posts, etc. Done to death gazillion times in various threads, as well as 'how to' stickies. I just mentioned the few main things you should bear in mind.
200hrs modular CPL in CASA system or 150hr 'integrated' may not meet the EASA CPL requirements, due to some instrument and night training as well. Plus the 100hrs solo/buddies jolly flights if lowtimer (ie not PIC on comm ops, instructing etc in other system)