Play a LOT and level up quickly, leaving all weapons at their current level.
Buy/win/gain new ones which are on your level as they become avaliable (lots of play means lots of coins). Getting "forced" to swap weapons now and then will only make you better player and broaden your skills.
When level 55, have patience. Upgrade one weapon at a time and load it up with your best mods. When you're already at max level (for now), a single cost of 70/125/170 gems (those are the final upgrade prices for Epic/Legendary/Mythical) is still a lot cheaper than most weapons were in initial purchase price before.
^This strategy really only helps extreme newcomers, the new upgrade prices are still bogus if you'd paid for a weapon with gems before the update, and now have to shell out 90 more gems for an upgrade. Just imagine all of those p2w kids having to beg their moms for even more money to throw at this app.
I had quite a few maxed out weapons, including Mythical in every category, when the new system came to be.
I was lvl 40 then, and my maxed out weapons became lvl 45. I did it the way I suggest above, albeit with a couple of upgrades of favourite weapons along the way. But mostly I got new ones and used them alongside my 45s.
Upgrading every weapon every time you level up is not a good idea, and highly unnecessary! The cost is the same regardless of how many levels, although it increases with the level you reach, so do it in leaps and bounds, or wait until you reach 55 to minimize the cost.
I didn't exactly "need" any new weapons, but buying new ones at a high level will get you guns at high level for coins only, unless you already have ALL of them, and lets you save up on gems until you can finalize your armory with a single upgrade for each weapon.
I still don't have a 55 in each category, but at least one 50+ in each and a couple of 55s.
That still leaves the whole "paid for a weapon in gems prior to the update and now you must pay more gems for it to be viable or ditch your effort" thing wide open. GG on your weapon-gaining strategy though, I think you should make a cheeky strategy page on the wiki for it, especially with the new pg3d economy.
Yeah. I was a bit grumpy to begin with as well, especially since I kind of expected getting banned for syncing two devices, but apparently I did it right! (They were both mine and I paid money on both).
Then I got playing and realised that I could still do some damage!
I have an entry about Modules in the works as well. I've gotten 1200 of those so far, of every kind but one, and have pictures of the filled in help section with all of the special abilities but one. (I'm missing a Melee Mod, not elemental, not Alloy or Shiv).
Problem is that I can't even get the formatting right for a forum post... and I'm even worse when it comes to Wikis and such! So I'll probably just write it up as a thread here and let someone who knows the ways of the Wiki transfer the info there.