General advice please.

I've just about done enough to unlock the next campaign but I'm far of getting perfect and completing some of the mission

In my opinion if you made it through all the current campaigns, your best choice is to wait.

Currently people with all the medals accumulated through the campaigns, achievements and army groups have solid rosters for both allied and axis.

This means no potential for easytech to gain more money with future updates if it's going to be just adding campaigns and army groups like it happened in last patch.

Maybe some tweaking of a few currently existing generals occur in the upcoming patches as new generals and new skills are added, or tweaking of current ones (remember when for the game prior to this one they added on a patch that forest fighting/mountain fighting/desert fighting did also cause no movement penalty in that tile, thus buffing those generals that had them?)

Chinese generals may be added on next patch. And it would not be a bad choice to rebrand axis and allies into wto and nato for future campaign additions, thus many (specially of axis) generals not showing in the general list (but you can add your best to the 3 empty spots they give you), thus forcing you to buy new generals for these campaigns.

Who knows? Precisely because of this is why I suggest waiting as there's no advantage of buying more now if you already made it over the five campaigns.

I think we should have a few guides here and there. If not, you can open a new thread and ask for help for Campaigns.

The Allied side can be cleared without buying any gens yet. I think you'd need to have lvl 2-3 inf, tank and arty tech to do so.

Rokossovsky is similar to Montgomery. His *s are slightly lacking, as compared to the British. However, he has Anti-Armour which is better than as Monty's Ace Forces. Either ways, Rokossovsky is a good buy.

When compared to his peer, Vatutin, Rokossovsky's output is overall consistently lower if Panzer Leader doesn't activate often due to Vatutin's possession of an extra tank * and Tide of Iron. Depending on circumstances, Vatutin's Desperate can vary from being an important skill that is a slightly worse version of Rumour, to a very low utility skill when he obliterates all opposition. Rokossovsky's 2 movement *s give him a slight mobility edge.
Overall, Rokossovsky remains a good buy as a mobile enemy wave destroyer.

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I need the opinion of members who have cleared 1944 Western Front. If the Soviets do not appear there, then, investing in Soviet gens may be less appealing.
In this case, I'd urge you to pick Monty instead, if you really need him now.