Indeed there are many opinions of what can be done differently. Me prefering to stick close to OTL developments, i'll adress some ideas in no particular order.
Su-24 development should have been focused on the VG variant from the start. So that means the T-6 prototype flies with VG wings already in 1967, and subsequent prototypes are brought forward. That might save perhaps 2 years of development, so it enters service in 1972 instead of 1974. Most importantly, from the late seventies at least it should be exported to anyone willing to pay, thus bringing more money to USSR. It will be a formidable bomb truck and be used in the Iraq- Iran war as well as Syria-Israel clashes etc.
MiG-23 should be the same as above, ie don't waste time with the useless lift engine MiG-23PD variant. So that means more design resources focused on sorting out the MiG-23 issues earlier. Also very importantly, don't export such a monkey model as MiG-23MS that damaged the credibility of soviet weapons. The need for secrecy is understandable, but the export MiG-23 needs to have at least a major advance to make it worthwhile ie longer range radar and BVR missiles. So the export MS should have a downgraded S-23E radar based on the early S-23L model, without LD/SD capability if necessary, and downgraded but still BVR R-23 missiles.
Su-27 could also greatly benefit from having the useless T-4 project cancelled (certainly no prototype should be built) and all freed resourced diverted to it. Also the project needs to be accelerated since the F-15 flew in 1972 and in service in 1976. Even if the Su-27 can fly a year earlier in 1976 and entered limited service in 1980-81 in an initial variant would be a great advance, but they also need to avoid the costly redesign, ironically they have looked initially at many of the features of the future T-10S, ie the widely spaced fins, dorsal airbrake, LE flaps etc. So if the T-10 prototype is closer to the T-10S as eventually built it would be a step forward, the redesign won't have to be so extensive and time-consuming. Lyulka would have to also step up his game and get the engine ready sooner, as i understand considerable time was lost trying to build an engine similar to the F-100 and abandoning the initial configuration proposed, only to more or less return to the initial configuration but with some features from the RD-33. Then the T-10S would fly in 1980 and enter service in 1984.
Again, critical to offer the Su-27 for export as soon as ready, say from 1985 so that as much money can be made as possible.
Also, the new generation MiG-29/31, Su-27, Su-24 should all have IFR systems from the start, and perhaps some of the older MiG-23, Su-17, Su-15, MiG-25 etc. should be retrofitted with IFR systems. Cettainly IFR system would be appreciated by the iraqis during the Iran-Iraq war, among others.