Pilot reported difficulty in contacting SoCal Approach during arrival to UDD. Pilot stated the increased traffic volume and clearance requests in the area is a concern and offered suggestions for what could be done to mitigate the situation.

Narrative

While descending via the SIZLR3 Arrival into UDD; it took approx 7 minutes for Palm Springs Approach [SoCal Approach] to acknowledge my check in. A [commercial] aircraft was apparently executing practice MAP's at PSP and not answering many radio calls; thus tying up the Controllers from handling my and other aircraft. The PSP; TRM; UDD triad of airports is a disaster waiting to occur. TRM and UDD are uncontrolled but long past the point each should have a Tower controlling the airspace at this time of year (winter/spring break). I have flown for a decade out of ZZZ. I can handle busy airspace. The Palm Springs area is a complete mess. One reason for the mess; is that to obtain an IFR clearance from either UDD or TRM you have a choice; contact 120.22 IF they can hear you or call the 1-800 number; which is perpetually busy.Jets are launching VFR and forcing ATC to issue IFR clearances in the air. That means there are numerous jets in the air with little or no real ATC services yet. This dominos downward and further delays the airplanes on the ground waiting and 'following the rules'. There can be only one reason for all this; ATC manpower is woefully inadequate for the volume of aircraft at this location; at this time of year; spring break and winter break in Palm Springs. UDD is a separate issue as it has numerous single engine aircraft; not communicating but flying all over the area at low altitude. I had Aircraft Y single prop fly right across my approach path while I was self announcing my way around the pattern at UDD. TRM and UDD are in dire need of at least a part time Control Tower each; before someone has a middair.Lack of adequate ATC manpower.Stop letting jets depart VFR and pick up IFR clearances in the air from TRM and UDD during the winter months and especially weekends and spring break winter months. Open more Approach/Departure control freqs; increase the number of Approach Controllers and increase the avenues for airplanes to obtain clearances on the ground at TRM and UDD. Having them 'forcing' their way into the air by departing VFR and 'picking up' IFR in the air is dangerous given the heightened volume of traffic during the times referenced above. I have personally had to call the 1-800 number well over 30 times in a row on multiple occasions in the past 3 weeks to obtain an IFR clearance out of TRM or UDD. Its a mess.