Definitely go to Frontier. 4 years a long time for a lot of things to go wrong and you can't expect Delta, or any major, to reward your loyalty if you stay at Endeavor. I think we all learned that a few years back when they dropped the PiC requirement and started hiring junior FOs because senior Captains with 10,000 hours flying their routes with their passengers were "untrainable"
The myth of Upgrade and get your PiC time and wait your turn has been proven completely false. You are on your own for your own career progression. The new more successful route seems to be upgrade, get your PiC, go somewhere to get a new type rating and then make a decision from multiple offers.
The move to Frontier seems to have no downside for you; more money, faster upgrade, still live in base, and a business model that has proven more stable in a downturn. You still have the option to apply to Delta and now with a A320 type you are more marketable than just another RJ Captain. You could actually end up at Delta sooner.
That being said I would advise you not to come to Frontier if you are not prepared to make it your final stop or to just use it as a stepping stone.